Enzymes and methods for processing lignin and other aromatic compounds

ABSTRACT

Enzymes for depolymerizing lignin. The enzymes include dehydrogenases, β-etherases, and glutathione lyases. The dehydrogenases can comprise one or more or LigD, LigO, LigN, and LigL. The β-etherases can comprise one or more of LigE, LigF, LigP, and BaeA. The glutathione lyases can comprise any one or more of LigG and a number of non-stereospecific, optionally recombinant glutathione lyases derived from Sphingobium sp. SYK-6, Novosphingobium aromaticivorans, Escherichia coli, Streptococcus sanguinis, Phanerochaete chrysosporium, and other microorganisms. The enzymes can be combined in compositions and/or used in methods of processing lignin or other aromatic compounds in vitro.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-FC02-07ER64494 awarded by the US Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to the enzymatic depolymerization of lignin and the enzymatic processing of other aromatic compounds.

BACKGROUND

Lignin provides structural rigidity in terrestrial plants and is largely comprised of guaiacyl and syringyl monoaromatic phenylpropanoid units that are covalently linked together in a purely chemical radical coupling polymerization process. The most prevalent type of inter-unit linkage between units is the β-ether linkage in so-called β-ether units making up the lignin polymer.

Because lignin is rich in aromatics, lignin can potentially serve as a source for a number of valuable aromatic polymers, oligomers, and monomers. However, lignin is notoriously difficult to process or depolymerize into simpler compounds.

A number of chemical methods for depolymerizing lignin are known, but these methods tend to involve high temperatures or pressures, expensive catalysts, and organic solvents. Tools and methods of depolymerizing lignin that avoid at least some of these drawbacks are needed.

SUMMARY OF THE INVENTION

The invention at least in part is directed to an enzymatic system that catalyzes β-ether cleavage of actual lignin in vitro with the recycling of cosubstrates NAD⁺ and GSH. In an exemplary version, the system uses the known LigD, LigN, LigE, and LigF enzymes from Sphingobium sp. strain SYK-6, plus a novel, non-stereospecific glutathione transferase from Novosphingobium aromaticivorans DSM12444 (NaGST_(Nu)). BaeA can be used in addition to or in place of LigE. A glutathione reductase from Allochromatium vinosum DSM180 (AvGR) is used to recycle the cosubstrates. The depolymerization of actual lignin with these enzymes is illustrated in FIGS. 1 and 16. The enzymatic depolymerization of lignin provided herein has several advantages over chemical routes as it (1) does not require high temperatures or pressures; (2) does not require expensive catalysts; (3) could be performed in an aqueous environment, eliminating the need for solvents (and subsequent separation/recycle); and (4) results in a well-defined set of aromatic monomers that have not undergone chemical transformations, and hence are more amenable for downstream processing (i.e., upgrading).

More generally, the invention encompasses methods of processing lignin. One method of the invention comprises contacting lignin comprising β-O-4 ether (β-ether) linkages in vitro with a dehydrogenase, a β-etherase, and a glutathione lyase. The dehydrogenase preferably comprises at least one of LigD, LigO, LigN, and LigL. The β-etherase preferably comprises at least one of LigE, LigF, LigP, and BaeA. The glutathione lyase preferably comprises at least one of LigG and a non-stereospecific glutathione lyase comprising an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1).

In some versions, the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of: asparagine or a conservative variant of asparagine at a position corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu)); threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)); asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)); glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)); lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)); isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)); glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)); serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)); tyrosine or a conservative variant of tyrosine at a position corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu)); arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and tyrosine or a conservative variant of tyrosine at a position corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

In some versions, the contacting occurs in the presence of a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG). The GSH reductase in some versions comprises an amino acid sequence at least 95% identical to SEQ ID NO:38 (AvGR).

Another method of the invention is a method of chemical conversion. The chemicals converted in the method preferably comprise aromatic chemicals. One method comprises contacting a first compound in vitro with a non-stereospecific glutathione lyase to yield a second compound. The non-stereospecific glutathione lyase may comprise any of those described above or elsewhere herein but preferably comprises a non-stereospecific glutathione lyase having an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1). The first compound preferably has a structure of Formula I or a salt thereof:

wherein R¹, R², and R³ are each independently —H, —OH, —O-alkyl, —O-lignin, or -lignin; R⁴ is —H, —OH, —SH, —COOH, —SO₃H, or —O-lignin; and SG is glutathione bound in an S or R configuration. The second compound has a structure of Formula II or a salt thereof:

wherein R¹, R², R³, and R⁴ are as defined above.

The invention also encompasses compositions. The compositions may include any of the components employed in the methods described herein. One composition of the invention comprises lignin comprising β-O-4 ether linkages, a dehydrogenase, a β-etherase, and a glutathione lyase. The dehydrogenase, a β-etherase, and a glutathione lyase preferably include any of those described above or elsewhere herein. In some versions, the composition further comprises a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG). The GSH reductase in some versions comprises a sequence at least about 95% identical to SEQ ID NO:38 (AvGR).

The invention also encompasses recombinant enzymes. The recombinant enzymes may include recombinant versions of any of the enzymes described above or elsewhere herein. The recombinant enzymes preferably include a recombinant non-stereospecific glutathione lyase. The non-stereospecific glutathione lyase may comprise any of those described above or elsewhere herein but preferably comprises a non-stereospecific glutathione lyase having an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1). The recombinant non-stereospecific glutathione lyase preferably comprises at least one non-native modification selected from the group consisting of an amino acid addition, an amino acid deletion, and an amino acid substitution.

The invention also encompasses processed lignin or compounds obtained through any of the methods described herein.

The objects and advantages of the invention will appear more fully from the following detailed description of the preferred embodiment of the invention made in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 shows the sphingomonadales β-etherase pathway, used to break the β-aryl ether ((3-O-4) bond of compounds such as guaiacylglycerol-β-guaiacyl ether (GGE). Names of enzymes catalyzing each reaction are taken from Sphingobium sp. SYK-6 (LigLNDOFEPG; Masai et al. 2003, Sato et al. 2009; Tanamura et al. 2011), Novosphingobium sp. MBESO4 (GST3) (Ohta et al. 2015), or Novosphingobium aromaticivorans (NaGST_(Nu)) (Example 1); the color of each enzyme name matches the arrow color of the reaction it catalyzes. Below each enzyme is listed which of the three sphingomonads investigated in Example 1 (Novosphingobium sp. PP1Y, Novosphingobium aromaticivorans, and Sphingobium xenophagum) are predicted to contain that enzyme. The α, β, and γ carbons of GGE are labeled in the topmost molecule, and the stereochemical designations of all chiral molecules are shown. As shown, metabolism of GGE begins with the stereoselective, NAD⁺-dependent oxidation (by LigD, LigO, LigL, LigN) of the β-aryl alcohol into the α-ketone, β-(2-methoxyphenoxy)-γ-hydroxypropiovanillone (MPHPV) (Sato et al. 2009). β(S)- and β(R)-MPHPV are then cleaved by stereospecific glutathione (GSH)-dependent β-etherases (e.g., LigF, LigE/P) to yield the β(R)- and β(S)-stereoisomers of the glutathione conjugate β-glutathionyl-γ-hydroxypropiovanillone (GS-HPV) and guaiacol (Masai et al. 2003, Gall and Kim et al. 2014). GSH-dependent enzymes that remove the glutathione moiety from GS-HPV to form hydroxypropiovanillone (HPV) and glutathione disulfide (GSSG) have been identified in vitro: LigG reacts specifically with β(R)-GS-HPV (Masai et al. 2003), whereas GST3 and NaGST_(Nu) react with both the β(R)- and β(S)-stereoisomers (Ohta et al. 2015).

FIGS. 2A and 2B show cell densities and extracellular metabolite concentrations of N. aromaticivorans cultures grown in SMB containing 3 mM GGE (FIG. 2A, panels A-F), or 4 mM vanillate and 1.5 mM GGE (FIG. 2B, panels G-L). Data are shown for strains 12444Δ1879 (panels A,B,G,H), 12444Δ2595 (panels C,D,I,J), and 12444ecyghU (panels E,F,K,L). The y-axes of panels H, J, and L use different scales. For comparison, cell density data for cultures grown in SMB containing 4 mM vanillate only are included in panels G, I, and K.

FIG. 3 shows growth and extracellular metabolite levels in a representative culture of Novosphingobium aromaticivorans 12444Δ1879 in SMB containing 200 μM GGE. The rate of GGE metabolism by N. aromaticivorans (˜200 μM within ˜30 h) is comparable to that of Erythrobacter sp. SG61-1L (˜200 μM within ˜60 h, though this strain apparently cannot further metabolize guaiacol; Palamuru et al. 2015) and Novosphingobium sp. MBESO4 (˜900 μM within ˜40 h, though this strain apparently cannot assimilate carbon from GGE into cell material; Ohta et al. 2015).

FIGS. 4A and 4B show cell densities and extracellular metabolite concentrations from cultures of Novosphingobium sp. PP1Y (FIG. 4A, panels A,B,E,F) and Sphingobium xenophagum (FIG. 4B, panels C,D,G,H) grown in SMB containing 3 mM GGE (FIGS. 4A and 4B, panels A,B,C,D); or 4 mM vanillate (FIG. 4A, panels E,F) or glucose (FIG. 4B, panels G,H) with 1.5 mM GGE. The y-axis segments of panels D,F,H of FIGS. 4A and 4B are at different concentration scales. For comparison, cell density data for cultures grown in SMB containing 4 mM vanillate only are included in panels.

FIG. 5 shows an amino acid sequence alignment of various exemplary non-stereospecific glutathione lyases of the invention. The aligned glutathione lyase sequences include those of NaGST_(Nu) (SEQ ID NO:18), SYK6GST_(Nu) (SEQ ID NO:22), ecYghU (SEQ ID NO:26), ecYfcG (SEQ ID NO:30), ssYghU (SEQ ID NO:32), GST3 (SEQ ID NO:34), and PcUre2pB1 (SEQ ID NO:36). The NaGST_(Nu), ecYghU, ssYghU, ecYfcG, and PcUre2pB1 proteins have been structurally characterized. Residues from structurally solved proteins predicted to interact with GSH or GSSG molecules indicated with “*” and correspond to the following residues in SEQ ID NO:18 (NaGST_(Nu)): Asn25, Thr51, Asn53, Gln86, Lys99, Ile100, Glu116, Ser117, and Arg177. Residues predicted to be involved in the reaction mechanism are indicated with “#” and correspond to the following residues in SEQ ID NO:18 (NaGST_(Nu)): Tyr166, Tyr224, and Phe288. Alignment was made using MAFFT version 7 (mafft.cbrc.jp) (Katoh et al. 2002) in MegAlign Pro, which is part of the Lasergene 14.0 suite (DNASTAR, Madison, Wis.).

FIG. 6 shows kinetics of the conversion of the β(R)- (A) and β(S)- (B) stereoisomers of GS-HPV into HPV. Reactions used 8 nM NaGST_(Nu), 195 nM ecYghU, 195 nM ecYfcG, or 47 (A) or 18 (B) nM SYK6-GST_(Nu). The lines are non-linear least squares best fits to the experimental data using the Michaelis-Menten equation.

FIG. 7 shows time courses for the reaction of cell extracts from N. aromaticivorans strains 12444Δ1879 (A) and 12444Δ2595 (B) with a racemic sample of β(R)- and β(S)-MPHPV. The red dotted line in panel B indicates the time at which recombinant NaGST_(Nu) and additional GSH were added to the reaction.

FIG. 8 shows the structure of NaGST_(Nu) (pdb 5uuo). (A) Domain structure of one subunit of the homodimer. The GST1 N-terminal (thioredoxin) domain extends from Val39 to Gly129 (green), the GST2 C-terminal domain extends from Ser135 to Leu257 (maroon), and an extension of the C-terminal extends from Val258 to Phe288 (orange). (B) Residue contacts to the GSH dithiol (60% occupancy) and GS-SG (40%) in the NaGST_(Nu) 5uuo structure. The carbon atoms of GSH1 are colored light cyan; those of GSH2 are dark cyan; those of GS-SG are light orange. NaGST_(Nu) residues with 3.2 Å or shorter contacts to either GSH1 or GSH2 are labeled, and colored according to domain origin defined above. Selected distances between interacting atoms are shown.

FIG. 9 shows a comparison of the active site in closely related Nu-class GSTs. (A) Alignment of subunits of NaGST_(Nu) (pdb 5uuo; open form, white; closed form, blue), ecYghU (pdb 3ec8; orange), and ssYghU (pdb 4mzw; green). (B) Spatially conserved positions (Phe82, Tyr224 and Lys262) in the open form subunit of NaGST_(Nu) that define a triangle over the entrance to the active site used to approximate the size of the channel opening. (C) Positions (Phe82, Tyr224 and Phe288) and triangle defined in the closed form of NaGST_(Nu). (D) Positions (Gly83, Tyr225 and Gly263) and triangle defined in ecYghU. (E) Positions (Met80, Tyr222 and Ala255) and triangle defined in ssYghU.

FIG. 10 shows: (A) Molecular docking and energy-minimized positions of (R)- and (S)-GS-HPV (orange and purple lines, respectively) in the outwardly branching entrance to the active site of NaGST_(Nu) (the sulfur atom of GSH1 in the active site is visible as a yellow sphere); (B) Predicted residue interactions with (R)-GS-HPV; and (C) Predicted residue interactions with (S)-GS-HPV.

FIG. 11 shows a proposed mechanism for NaGST_(Nu)-catalyzed cleavage of either (R)- (left column) or (S)- (right column) thioether bonds in GS-HPV. (A) SG1 is close to the thiol of GHS2. Conserved Thr51, which lies within 3.0 Å of SG1, provides a hydrogen bond that promotes attack of SG1 on SG2 and formation of G1S-SG2. (B) Rupture of the thioether bond is facilitated by Y166, which stabilizes a transient enolate intermediate. (C) Collapse of the enolate to the observed products.

FIG. 12 shows modeling of substrates into the active sites of NaGST_(Nu) and ecYghU. Panels (A) and (B) show modeling of β(R)- and β(S)-GS-HPV into NaGST_(Nu). Panels (C) and (D) show modeling of β(R)- and β(S)-GS-HPV into ecYghU. Panels (E) and (F) show modeling of β(R)- and β(S)-GS-conjugated syringyl phenylpropanoids into NaGST_(Nu). Coloring for NaGST_(Nu) is the same as in FIG. 8 (with residues for ecYghU in parentheses): E4 (T5) to P38 (P39) in gray; V39 (V40) to G129 (G130) in green; V130 (Y131) to T134 (Q135) in gray; S135 (D136) to L257 (1257) in maroon; V258 (V258) to F288 (G288) in orange. Residues predicted to be involved in catalysis of the glutathione lyase reaction are Tyr66 and Tyr224 (Tyr167 and Tyr225 in ecYghU). Resides that contribute to differences in active site channel interiors between NaGST_(Nu) and ecYghU are Phe82 and Phe288 in NaGST_(Nu), and Arg260 and Asn262 in ecYghU. Carbon atoms of GSH1 are yellow, and those of the GS-conjugated substrates (GS-HPV or the syringyl analogue) are cyan.

FIG. 13 shows a phylogenetic analysis of Nu-class glutathione-S-transferases. BLAST searches of the NCBI non-redundant protein database were performed using NaGST_(Nu) and GST3 as queries. The top 5,000 hits from both of these searches were collected; every fifth member from each of these sets was transferred into a new combined set of 2,000 proteins. Sequences for SYK6GST_(Nu) and ecYfcG were added to the combined set, to give a set of 2,002 proteins. Proteins in this set were aligned using MAFFT in MegAlign Pro, which is part of the Lasergene 14.0 suite (DNASTAR, Madison, Wis.). A phylogenetic tree was calculated via the maximum likelihood method in RAxML v8.2.3 (Stamatakis, 2014), using 100 rapid bootstrap inferences. The tree was visualized using Interactive Tree of Life v3 (http://itol.embl.de). Enzymes experimentally reported here (NaGST_(Nu), SYK6GST_(Nu), ecYghU, ecYfcG) or elsewhere (GST3 (Ohta et al., 2015)) to be able to convert GS-HPV into HPV are identified.

FIG. 14 shows percent methanol in the running buffer during HPLC analysis as used in the experiments in Example 1. The remainder of the running buffer was Buffer A (5 mM formic acid, 5% acetonitrile in H₂O), and the flow rate was 1 mL/minute.

FIG. 15 shows absorbance (280 nm) and retention times of metabolites identified by HPLC as described in Example 1.

FIG. 16 shows aromatic monomers and the β-etherase pathway. Panel (A) shows the structures of predominant monomeric phenylpropanoids found in lignin, guaiacyl (G, in blue), syringyl (S, in red), as well as tricin (T, in green) units. Arrows indicate where inter-unit linkages are formed during radical coupling reactions. Dashed lines indicate positions that may form additional covalent bonds during post-coupling reaction mechanisms. Panel (B) shows β-etherase pathway-mediated degradation of the diaromatic β-ether-linked model compound GGE via NAD⁺-dependent dehydrogenases LigD and LigN to form GGE-ketone (also referred to herein as “β-(2-methoxyphenoxy)-γ-hydroxypropiovanillone” or “MPHPV”) and NADH. GGE-ketone undergoes GSH-dependent β-ether cleavage by β-etherase enzymes LigE and LigF to yield guaiacol and GS-HPV as monoaromatic derivative products. GS-HPV undergoes GSH-dependent thioether cleavage by NaGST_(Nu) or LigG, producing GSSG and monoaromatic product HPV. As indicated by the dashed arrows, AvGR recycles co-substrates GSH and NAD⁺ via NADH-dependent reduction of GSSG. For reactions involving an R- or S-configured epimer as the substrate, the isomer towards which each enzyme exhibits activity is shown in grey text. Panel (C) shows how β-etherase pathway enzymes degrade GTE through intermediate GTE-ketone to yield tricin and HPV.

FIGS. 17A and 17B show HPLC chromatographic traces of substrates and products of β-etherase pathway assays using NAD⁺ (2.0 mM), GSH (4.0 mM), and erythro-GGE (6.0 mM). Elution time of compounds (absorbance at 280 nm) are highlighted by shading: NAD⁺ and NADH (˜3.0 min), GS-HPV (˜4.5 min), HPV (˜9.0 min), guaiacol (˜14.0 min), erythro-GGE (˜18.0 min), threo-GGE (˜19.0 min), and GGE-ketone (˜21.5 min). Structures of GS-HPV, HPV, guaiacol, GGE, and GGE-ketone are shown in FIG. 16 (B). Panel (A) of FIG. 17A shows a control sample to which no enzymes were added. After 4 h incubation with one of the following combinations of enzymatic catalysts (50 μg/mL each): the remaining panels in FIGS. 17A and 17B show products in assays containing (FIG. 17A, panel B) LigDNEF and NaGST_(Nu); (FIG. 17A, panel C) LigDNEF, NaGST_(Nu), and AvGR; (FIG. 17B, panel D) LigDNEF and LigG; and (FIG. 17B, panel E) LigDNEF, LigG, and AvGR.

FIG. 18 shows time-dependent changes in concentrations of erythro-GGE (

), threo-GGE (

), GGE-ketone (-o-), HPV (

), and guaiacol (

) in an assay that, at 0 min, was supplemented with NAD⁺ (2.0 mM), GSH (4.0 mM), and erythro-GGE (6.0 mM), as well as (50 μg/mL each) LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR. Numbers in parentheses represent the measured concentration (mM) of each compound after 4 h of incubation. Structures of HPV, guaiacol, GGE, and GGE-ketone are shown in FIG. 16 (B). erythro-GGE is a mixture of enantiomers (αR,βS)-GGE and (αS,βR)-GGE. threo-GGE is a mixture of enantiomers (αS,βS)-GGE and (αR,βR)-GGE.

FIGS. 19A and 19B show HPLC chromatographic traces of β-etherase pathway products in reactions containing the indicated enzymes and NAD⁺ (5.0 mM), GSH (5.0 mM), and GTE (1.0 mM, a 6:1 mixture of erythro-GTE to threo-GTE). Elution time of compounds (absorbance at 280 nm) are highlighted: NAD⁺ and NADH (˜3.0 min), GS-HPV (˜5.5 min), HPV (˜16.5 min), tricin (˜51.5 min), threo-GTE (˜52.0 min), erythro-GTE (˜56.0 min), and GTE-ketone (˜58.5 min). Panel (A) of FIG. 19A shows the control sample to which no enzymes were added. After 4 h incubation with one of the following combinations of enzymatic catalysts (50 μg/mL each): the panels in FIG. 19B show products in assays containing (panel B) LigD, LigN, LigE, LigF, and NaGST_(Nu); (panel C) LigD, LigN, LigE and LigF; (panel D) LigD and LigN. Structures of GS-HPV, HPV, tricin, GTE, and GTE-ketone are shown in FIG. 16 (C).

FIG. 20 shows analytical GPC traces (λ=200 nm) showing the size distribution of (A) unfractionated HP lignin (MW=8,665), (B) fractions of MCS lignin collected from preparative GPC, and (C) the fractions that were pooled and used as the substrate in enzyme assays: fraction 1 (MW=11,550), fraction 2 (MW=10,780), and fraction 3 (MW=9,340). For reference, the approximate MW of a 10-mer is indicated with a dashed line.

FIG. 21 shows HPLC chromatographic traces of coupled β-etherase pathway reactions supplemented with NAD⁺ (2.0 mM), GSH (4.0 mM), and HP lignin fractions (2.2 mg mL⁻¹). Elution times of compounds (absorbance at 280 nm) are highlighted: NAD⁺ and NADH (˜3.0 min), HPS (˜10.0 min), unknown (grey, ˜20.0 min), and syringaresinol (˜21.5 min). Panel (A) shows the pooled GPC fractions 1 (MW=11,550), 2 (MW=10,780), and 3 (MW=9,340) without enzyme addition. Panel (B) shows after 4 h incubation with LigD, LigN, LigE, LigF, NaGST_(Nu) and AvGR (50 μg/mL each) and pooled HP lignin fractions 1-3 as the substrate.

FIG. 22 shows analytical GPC traces (λ=200 nm) showing the distribution of (A) unfractionated MCS lignin (MW=5,980), (B) fractions of MCS lignin collected from preparative GPC, and (C) the fractions used as substrates in enzyme assays: fraction 1 (MW=10,710), fraction 5 (MW=5,370), fraction 8 (MW=1,390), fraction 14 (MW=660), and fraction 17 (MW=460). For reference, the approximate MW of a 10-mer is indicated with a dashed line.

FIGS. 23A and 23B show HPLC chromatographic traces of β-etherase pathway enzyme activities in reactions containing NAD⁺ (2.0 mM), GSH (4.0 mM), and MCS lignin or the indicated MCS lignin fractions (2.2 mg mL⁻¹). Elution times (absorbance at 280 nm) are highlighted by shading: NAD⁺ (˜3.0 min), HPV (˜9.0 min), HPS (˜10.0 min), unknowns (grey, ˜18.0-19.0 min), and tricin (˜26.5 min), and an unknown broad peak (orange, ˜22.0-29.0 min). Panel A in FIG. 23A shows the control sample (unfractionated by GPC) to which no enzymes were added. The remaining panels in FIGS. 23A and 23B show products after 4 h incubation with 50 μg/mL each of LigD, LigN, LigE, LigF, NaGST_(Nu) and AvGR, and one of the following MCS lignin fractions: (FIG. 23A, panel B) fraction 1 (MW=10,710), (FIG. 23A, panel C) fraction 5 (MW=5,3670), (FIG. 23B, panel D) fraction 8 (MW=1,390), (FIG. 23B, panel E) fraction 14 (MW=660), and (FIG. 23B, panel F) fraction 17 (MW=460). Structures of HPV, HPS, and tricin are shown in FIG. 16.

FIG. 24 shows the extracellular concentration of metabolites with growth of N. aromaticivorans strains 12444Δ1879 (effective wild-type; A), 12444ΔligE (B), 12444Δ2872 (C), 12444ΔligEΔ2872 (D), and 12444ΔligEΔ2873 (E) in Standard Mineral Base (SMB) containing 3 mM vanillate and 1 mM erythro-GGE. Extracellular concentrations of vanillate are not shown. The segments of panels A-E use different x-axis scales for clarity of presentation.

FIG. 25 shows stereoisomer(s) of MPHPV remaining in the media in samples from the final time point of the experiment shown for FIG. 24 for the 12444ΔligEΔ2872 and 12444ΔligEΔ2873 cultures (F-H and I-K, respectively). The final time point samples were combined with H₂O, recombinant LigF, or recombinant LigE to determine which stereoisomer(s) of MPHPV remained in the media. The two stereoisomers each of erythro-GGE, threo-GGE, MPHPV, and GS-HPV are not distinguishable in our method of analysis.

FIG. 26 shows reactions of racemic MPHPV with the Saro_2872 and Saro_2873 polypeptides individually and combined. Racemic (β(R) and β(S)) MPHPV was initially mixed with H₂O (A), or the Saro_2872 or Saro_2873 polypeptides individually (B,C), or combined (D). The samples containing both Saro_2872 and Saro_2873 was then split and combined with either LigE (Saro_2405) or LigF1 (Saro_2091). All reactions contain at least 5 mM GSH. The β(R) and β(S) stereoisomers of both MPHPV and GS-HPV are indistinguishable in our analysis.

FIG. 27 shows gel permeation chromatography of various BaeA (Saro_2872 and Saro_2873 heterodimer) variants and other standard proteins of known molecular weights.

FIG. 28 shows a phylogentic tree of LigE and LigF homologues and the Saro_2872 and Saro_2873 polypeptides.

FIG. 29 shows a polypeptide sequence alignment of Saro_2872 and Saro_2873 with known LigF enzymes.

DETAILED DESCRIPTION OF THE INVENTION

One aspect of the invention includes methods of processing lignin. The term “lignin” used herein refers to any compound comprising covalently linked phenylpropanoid units. Phenylpropanoids include compounds commonly referred to as “phenylpropane units,” “lignin monomer units,” or variants thereof, and are well-known compounds in the art. Phenylpropanoids include a substituted or non-substituted six-carbon aromatic phenyl group and a substituted or non-substituted three-carbon tail. The phenyl group and tail may be substituted or unsubstituted. The tail may be saturated or unsaturated. The substitutions on the phenyl group may include hydroxy and alkoxy (e.g., methoxy) groups, among others. The substitutions on the tail may include hydroxy, alkoxy, carboxy, thiol, and sulfonate groups, among others. Exemplary phenylpropanoid units include the p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units derived from p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, respectively. The phenylpropanoid units can be linked to other phenylpropanoid units, flavonoid units such as tricin, or other types of chemical units. The phenylpropanoid units are preferably linked through radical coupling. Exemplary linkages include β-O-4, 5-5, β-5, β-1, 4-O-5, and β-β linkages. See, e.g., Santos et al. 2013.

Two types of lignin include natural lignin and synthetic lignin. “Natural lignin” refers to lignin in which the phenylpropanoid units are covalently linked in nature (in vivo), regardless of whether or not the lignin is subsequently processed in vitro. Natural lignin encompasses lignin from non-genetically engineered plants as well as genetically engineered plants. The genetically engineered plants include plants that have been genetically engineered for altered lignin production, such as incorporation of ferulates moieties (U.S. Pat. Nos. 8,569,465 and 9,388,285), flavan-3-ols and/or gallic acid derivatives (U.S. Pat. No. 8,685,672), high syringyl content (see the examples), or other modifications (U.S. Pat. Nos. 9,487,794; 9,441,235; and 9,493,783). “Synthetic lignin” refers to lignin in which the phenylpropanoid units are covalently linked in vitro. Methods of covalently linking phenylpropanoid units in vitro through radical coupling reactions are well known in the art. See, e.g., Grabber et al. 1996.

“Processing” or grammatical variants thereof refers herein to modifying lignin in any manner to result in at least one structural change. Processing can occur through chemical, physical, or enzymatic methods. Examples of processing include depolymerization, oxidation, acid treatment, base treatment, enzyme treatment, heating, mechanical shearing, etc. The processing may depolymerize the lignin (at least to some degree), chemically modify the lignin, physically break apart the lignin, remove or add functional groups on the lignin, or result in other structural changes.

Certain methods of the invention are directed to enzymatically processing lignin comprising β-O-4 linkages (β-ether linkage) to break at least a portion of the β-O-4 linkages and/or release compounds from the lignin. The processing can advantageously be performed in vitro using one or more enzymes. The term “in vitro” in this context refers to processing with enzymes in which the enzymes are not actively produced by any intact, living organisms, and is contrasted with in vivo processing, in which the enzymes involved with the processing are actively produced by one or more intact, living organisms. Thus, in some versions, the enzymes involved in the processing are not actively produced during the duration of the processing. In some versions, the processing occurs in the absence of intact, living microorganisms. In some versions, the processing occurs in the absence of any intact, living Sphingobium species, Erythrobacter species, Novosphingobium species, Escherichia species, Streptococcus species, and/or Phanerochaete species.

In some versions, the enzymes involved in the processing are purified enzymes. The term “isolated” or “purified” means a material that is removed from its original environment, for example, the natural environment if it is naturally occurring, or a fermentation broth if it is produced in a recombinant host cell fermentation medium. A material is said to be “purified” when it is present in a particular composition in a higher or lower concentration than the concentration that exists prior to the purification step(s). For example, with respect to a composition normally found in a naturally-occurring or wild type organism, such a composition is “purified” when the final composition does not include some material from the original matrix. As another example, where a composition is found in combination with other components in a recombinant host cell fermentation medium, that composition is purified when the fermentation medium is treated in a way to remove some component of the fermentation, for example, cell debris or other fermentation products, through, for example, centrifugation or distillation. As another example, a naturally-occurring polynucleotide or polypeptide present in a living animal is not isolated, but the same polynucleotide or polypeptide, separated from some or all of the coexisting materials in the natural system, is isolated, whether such process is through genetic engineering or mechanical separation. In another example, a polynucleotide or protein is said to be purified if it gives rise to essentially one band in an electrophoretic gel or a blot.

A first step in the enzymatic processing involves contacting lignin comprising β-O-4 (β-ether) linkages with a dehydrogenase. The dehydrogenase is preferably capable of oxidizing α-hydroxyls on β-ether units to corresponding α-ketones. The term “β-ether unit” is used herein to refer to a phenylpropanoid moiety linked to a second phenylpropanoid moiety via a β-O-4 linkage. See, e.g., FIGS. 1 and 16. Exemplary enzymes used for this step include any one or more of LigD, LigO, LigN, and LigL. Exemplary LigD, LigO, LigN, and LigL enzymes include those from Sphingobium sp. SYK-6 having the sequences of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and SEQ ID NO:8, respectively, which are encoded by SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, and SEQ ID NO:7, respectively. LigD, LigO, LigN, and LigL enzymes are found in organisms other than Sphingobium sp. SYK-6 and can be used in place of or in addition to those from Sphingobium sp. SYK-6. Modified forms of the native LigD, LigO, LigN, and LigL enzymes can also suitably be used, provided the modified forms maintain the activity of the native enzymes. Such modified forms that maintain the activity of the native enzymes are also referred to herein as LigD, LigO, LigN, and LigL enzymes. The modified forms comprise sequences at least 95% identical to the amino acid sequences of the corresponding native enzymes.

The LigD and LigO enzymes from Sphingobium sp. SYK-6 have a specificity for α-hydroxyls in the α(R) stereochemical configuration. The LigN and LigL enzymes from Sphingobium sp. SYK-6 have a specificity for α-hydroxyls in the α(S) stereochemical configuration. To ensure efficient processing in a non-stereospecific manner, it is preferred that the lignin is contacted with at least one α(R) stereospecific enzyme and least one α(S) stereospecific enzyme. Accordingly, preferred combinations include one or more of LigD and LigO with one or more of LigN and LigL.

LigD, LigO, LigN, and LigL homologs from Erythrobacter sp. SG61-1L can react with all four possible stereoisomers of GGE (see Palamuru et al. 2015) and can be used in place of or in combination with the enzymes from Sphingobium sp. SYK-6.

A second step in the enzymatic processing involves contacting preprocessed (preliminarily processed) lignin, such as a product of the first step, with a β-etherase. The β-etherase is preferably capable of catalyzing glutathione-dependent β-ether cleavage to yield a β-glutathione-linked phenylpropanoid unit in place of the β-ether phenylpropanoid unit. See, e.g., FIGS. 1 and 16. The second step can be performed simultaneously with or subsequent to the first step. Exemplary enzymes used for this step include any one or more of LigE, LigF, LigP, and BaeA.

Exemplary LigE, LigF, and LigP enzymes include those from Sphingobium sp. SYK-6 having the sequences of SEQ ID NO:10, SEQ ID NO:12, and SEQ ID NO:14, respectively, which are encoded by SEQ ID NO:9, SEQ ID NO:11, and SEQ ID NO:13, respectively. LigE, LigF, and LigP enzymes are found in organisms other than Sphingobium sp. SYK-6 and can suitably be used in place of or in addition to those from Sphingobium sp. SYK-6. Modified forms of the native LigE, LigF, and LigP enzymes can also suitably be used, provided the modified forms maintain the activity of the native enzymes. Such modified forms that maintain the activity of the native enzymes are also referred to herein as LigE, LigF, and LigP enzymes. The modified forms comprise sequences at least 95% identical to the amino acid sequences of the corresponding native enzymes.

“BaeA” refers to a heterodimer of a first polypeptide having an amino acid sequence of SEQ ID NO:40 or an amino acid seqeuence at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or more identical thereto and a second polypeptide having an amino acid sequence of SEQ ID NO:42 or an amino acid seqeuence at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or more identical thereto. An exemplary BaeA is a heterodimer of Saro_2872 from Novosphingobium aromaticivorans (encoded by SEQ ID NO:39 and represented by SEQ ID NO:40) and Saro_2873 from Novosphingobium aromaticivorans (encoded by SEQ ID NO:41 and represented by SEQ ID NO:42). The second polypeptide preferably comprises an asparagine or a conservative variant of asparagine at a position corresponding to position 14 of SEQ ID NO:41 and/or a serine or a conservative variant of serine at a position corresponding to position 15 of SEQ ID NO:42.

The LigE and LigP enzymes from Sphingobium sp. SYK-6 and BaeA have a specificity for cleaving β-ether linkages in the β(R) stereochemical configuration. The LigF enzyme from Sphingobium sp. SYK-6 has a specificity for cleaving β-ether linkages in the β(S) stereochemical configuration. To ensure efficient processing in a non-stereospecific manner, it is preferred that the substrate is contacted with at least one β(R) stereospecific enzyme and least one β(S) stereospecific enzyme. Accordingly, preferred combinations include one or more of LigE, LigP, and BaeA with LigF.

LigE/P and LigF homologues from other organisms are also expected to be stereospecific. See for example Gall and Ralph et al. 2014. SLG_08660 (SYK-6 LigE), PP1Y_AT11664 (Novosphingobium sp. PP1Y), SLG_32600 (SYK-6 LigP), and Saro_2405 (Novosphingobium aromaticivorans) were all shown to be specific for β(R)-compounds. Likewise, SLG_08650 (SYK-6 LigF), Saro_2091 (Novosphingobium aromaticivorans), and Saro_2865 (Novosphingobium aromaticivorans) were all shown to be specific for β(S)-compounds.

A third step in the enzymatic processing involves contacting preprocessed lignin, such as a product of the second step, with a glutathione lyase. The glutathione lyase is preferably capable of cleaving glutathione from β-glutathione-linked phenylpropanoid units. The cleavage may use glutathione as a cosubstrate and produce glutathione disulfide in the process. See, e.g., FIGS. 1 and 16. The third step can be performed simultaneously with or subsequent to the second step and/or the first and second steps. Exemplary enzymes used for this step include any one or more of LigG from Sphingobium sp. SYK-6 having the amino acid sequence of SEQ ID NO:16 (encoded by SEQ ID NO:15), a Nu-class glutathione 5-transferase (GST) from Novosphingobium aromaticivorans DSM 12444 having the amino acid sequence of SEQ ID NO:18 (NaGST_(Nu)) (encoded by SEQ ID NO:17), a recombinant NaGST_(Nu) having the amino acid sequence of residues 21-313 of SEQ ID NO:20 (encoded by SEQ ID NO:19 prior to cleavage), a Nu-class GST from Sphingobium sp. SYK-6 having the amino acid sequence of SEQ ID NO:22 (SYK6GST_(Nu)) (encoded by SEQ ID NO:21), a recombinant SYK6GST_(Nu) having the amino acid sequence of residues 21-324 of SEQ ID NO:24 (encoded by SEQ ID NO:23 prior to cleavage), a Nu-class GST from Escherichia coli DH5α having the amino acid sequence of SEQ ID NO:26 (ecYghU) (encoded by SEQ ID NO:25), a recombinant ecYghU having the amino acid sequence of residues 21-313 of SEQ ID NO:28 (encoded by SEQ ID NO:27 prior to cleavage), a Nu-class GST from Escherichia coli DH5α having the amino acid sequence of SEQ ID NO:30 (ecYfcG) (encoded by SEQ ID NO:29), a Nu-class GST from Streptococcus sanguinis SK36 having the amino acid sequence of SEQ ID NO:32 (ssYghU) (encoded by SEQ ID NO:31), a Nu-class GST from Novosphingobium sp. MBESO4 having the amino acid sequence of SEQ ID NO:34 (GST3) (encoded by SEQ ID NO:33), and a Nu-class GST from Phanerochaete chrysosporium RP-78 having the amino acid sequence of SEQ ID NO:36 (PcUre2pB1) (encoded by SEQ ID NO:35).

The LigG from Sphingobium sp. SYK-6 has a specificity for β-glutathione-linked phenylpropanoid units with the glutathione moieties linked in the β(R) stereochemical configuration and is referred to herein as a “stereospecific glutathione lyase.” By contrast, the Nu-class glutathione S-transferase (GST) from Novosphingobium aromaticivorans DSM 12444 having the amino acid sequence of SEQ ID NO:18 (NaGST_(Nu)) (encoded by SEQ ID NO:17), the recombinant NaGST_(Nu) having the amino acid sequence of residues 21-313 of SEQ ID NO:20 (encoded by SEQ ID NO:19 prior to cleavage), the Nu-class GST from Sphingobium sp. SYK-6 having the amino acid sequence of SEQ ID NO:22 (SYK6GST_(Nu)) (encoded by SEQ ID NO:21), the recombinant SYK6GST_(Nu) having the amino acid sequence of residues 21-324 of SEQ ID NO:24 (encoded by SEQ ID NO:23 prior to cleavage), the Nu-class GST from Escherichia coli DH5α having the amino acid sequence of SEQ ID NO:26 (ecYghU) (encoded by SEQ ID NO:25), the recombinant ecYghU having the amino acid sequence of residues 21-313 of SEQ ID NO:28 (encoded by SEQ ID NO:27 prior to cleavage), the Nu-class GST from Escherichia coli DH5α having the amino acid sequence of SEQ ID NO:30 (ecYfcG) (encoded by SEQ ID NO:29), the Nu-class GST from Streptococcus sanguinis SK36 having the amino acid sequence of SEQ ID NO:32 (ssYghU) (encoded by SEQ ID NO:31), the Nu-class GST from Novosphingobium sp. MBESO4 having the amino acid sequence of SEQ ID NO:34 (GST3) (encoded by SEQ ID NO:33), and the Nu-class GST from Phanerochaete chrysosporium having the amino acid sequence of SEQ ID NO:36 (PcUre2pB1) (encoded by SEQ ID NO:35) are capable or predicted to be capable of cleaving β-glutathione-linked phenylpropanoid units with the glutathione moieties linked in either the β(R) or β(S) stereochemical configuration and are referred to herein as “non-stereospecific glutathione lyases.” To ensure efficient processing in a non-stereospecific manner, it is preferred that the substrate is contacted with at least one non-stereospecific glutathione lyase. Accordingly, preferred combinations include one or more of any of the non-stereospecific glutathione lyases described herein with or without LigG.

Other non-stereospecific glutathione lyases that can be used in place of or in addition to the non-stereospecific glutathione lyases described above include enzymes at least about 35%, at least about 40%, at least about 45%, at least about 50%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% identical to any one of SEQ ID NO:18 (NaGST_(Nu)), residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)), SEQ ID NO:22 (SYK6GST_(Nu)), residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)), SEQ ID NO:26 (ecYghU), residues 21-313 of SEQ ID NO:28 (recombinant ecYghU), SEQ ID NO:30 (ecYfcG), SEQ ID NO:34 (GST3); SEQ ID NO:32 (ssYghU), and SEQ ID NO:36 (PcUre2pB1). Such enzymes may be native enzymes or modified forms of native enzymes.

As discussed in the following examples, a number of residues of the non-stereospecific glutathione lyases described herein play at least some role in the enzymatic activity. See, e.g., FIG. 5. These include Asn25 of SEQ ID NO:18 (NaGST_(Nu)), Thr51 of SEQ ID NO:18 (NaGST_(Nu)), Asn53 of SEQ ID NO:18 (NaGST_(Nu)), Gln86 of SEQ ID NO:18 (NaGST_(Nu)), Lys99 of SEQ ID NO:18 (NaGST_(Nu)), Ile100 of SEQ ID NO:18 (NaGST_(Nu)), Glu 116 of SEQ ID NO:18 (NaGST_(Nu)), Ser117 of SEQ ID NO:18 (NaGST_(Nu)), Tyr166 of SEQ ID NO:18 (NaGST_(Nu)), Arg177 of SEQ ID NO:18 (NaGST_(Nu)), Tyr224 of SEQ ID NO:18 (NaGST_(Nu)), and corresponding residues in the other enzymes. A number of these residues are conserved across all the non-stereospecific glutathione lyases described herein. These include Thr51 of SEQ ID NO:18 (NaGST_(Nu)), Asn53 of SEQ ID NO:18 (NaGST_(Nu)), Gln86 of SEQ ID NO:18 (NaGST_(Nu)), Ile100 of SEQ ID NO:18 (NaGST_(Nu)), Glu 116 of SEQ ID NO:18 (NaGST_(Nu)), Arg177 of SEQ ID NO:18 (NaGST_(Nu)), and corresponding residues in the other enzymes.

Accordingly, suitable enzymes at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% identical to the non-stereospecific glutathione lyases described herein preferably comprise at least one, at least two, at least three, at least four, at least five, at least six, at least seven, or all of threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)), asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)), glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)), lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)), isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)), glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)), serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)), arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).

Suitable enzymes at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% identical to the non-stereospecific glutathione lyases described herein more preferably comprise at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of asparagine or a conservative variant of asparagine at a position corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu)), threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)), asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)), glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)), lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)), isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)), glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)), serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)), tyrosine or a conservative variant of tyrosine at a position corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu)), arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)), and tyrosine or a conservative variant of tyrosine at a position corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

Positions in a given enzyme that correspond to positions in a given sequence such as SEQ ID NO:18 (NaGST_(Nu)), SEQ ID NO:40 (Saro_2872), and SEQ ID NO:42 (Saro_2873), or any other sequence provided herein, can be identified through alignment of the sequence of the enzyme with the given sequence. A number of sequence alignment algorithms are known in the art. Exemplary sequence alignment algorithms include MAFFT version 7 (mafft.cbrc.jp) (Katoh et al. 2002) and Clustal W and other Clustal programs (Larkin et al. 2007). Other algorithms and programs are known in the art.

“Conservative variant” refers to residues that are functionally similar to a given residue such that one or more of the functionally similar residues may substitute for the given residue. Conservative variants of the standard amino acids are well known in the art. In some versions, aliphatic, non-polar amino acids (Gly, Ala, Ile, Leu, and Val) are conservative variants of one another. In some versions, aliphatic, polar amino acids (Cys, Ser, Thr, Met, Asn, Tyr and Gln) are conservative variants of one another. In some versions, aromatic amino acids (Phe, Tyr, Trp, and His) are conservative variants of one another. In some versions, basic amino acids (Lys, Arg, and His) are conservative variants of one another. In some versions, acidic amino acids (Asp and Glu) are conservative variants of one another. In some versions, an amino acid with an acidic side chain, Glu or Asp, is a conservative variant of its uncharged counterpart, Gln or Asn, respectively; or vice versa. In some versions, each of the following groups contains other exemplary amino acids that are conservative variants of one another: Ala and Gly; Asp and Glu; Asn and Gln; Arg and Lys; Ile, Leu, Met, and Val; Phe, Tyr, and Trp; Ser and Thr; and Cys and Met.

A fourth step in the enzymatic processing involves conducting the first three steps in the presence of a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG). The GSH reductase regenerates the GSH cosubstrate of the second and third steps from the GSSG co-product of the third step and regenerates the NAD⁺ cosubstrate of the first step from the NADH co-product of the first step. See, e.g., FIGS. 1 and 16. An exemplary GSH reductase is the GSH reductase from Allochromatium vinosum DSM180 (AvGR) having the sequence of SEQ ID NO:38 (encoded by SEQ ID NO:37). Other GSH reductases are known in the art and can be used in this step. Modified forms of the native AvGR and other suitable enzymes can also suitably be used, provided the modified forms maintain the activity of the native enzymes. The modified forms comprise sequences at least 95% identical to the amino acid sequences of the corresponding native enzymes.

The enzymatic processing outlined above is capable of depolymerizing lignin and/or releasing compounds therefrom. Compounds that are capable of being released from lignin include monomeric phenylpropanoid units and monomeric flavones. Examples of monomeric phenylpropanoid units capable of being released from lignin include monomeric guaiacyl phenylpropanoid units, monomeric syringyl phenylpropanoid units, and monomeric p-hydroxyphenyl phenylpropanoid units. Examples of flavones capable of being released from lignin include monomeric tricin units.

The lignin subjected to the enzymatic processing outlined above may have any of a number of average molecular weights (MW). The average molecular weight (MW) in some versions, for example, may be at least about 100, at least about 200, at least about 300, at least about 400, at least about 500, at least about 600, at least about 700, at least about 800, at least about 900, at least about 1000, at least about 1,250, at least about 1,500, at least about 1,750, at least about 2,000, at least about 2,500, at least about 3,000, at least about 3,500, at least about 4,000, at least about 4,500, at least about 5,000, at least about 6,000, at least about 7,000, at least about 8,000, at least about 9,000, at least about 10,000, at least about 11,000, at least about 13,000, at least about 15,000, or more. The average molecular weight (MW) in some versions may be up to about 150, up to about 200, up to about 300, up to about 400, up to about 500, up to about 600, up to about 700, up to about 800, up to about 900, up to about 1000, up to about 1,250, up to about 1,500, up to about 1,750, up to about 2,000, up to about 2,500, up to about 3,000, up to about 3,500, up to about 4,000, up to about 4,500, up to about 5,000, up to about 6,000, up to about 7,000, up to about 8,000, up to about 9,000, up to about 10,000, up to about 11,000, up to about 13,000, up to about 15,000, up to about 20,000 or more.

Certain methods of the invention are directed to methods of chemical conversion. Such methods may comprise contacting a first compound in vitro with a non-stereospecific glutathione lyase to yield a second compound.

The non-stereospecific glutathione lyase used in the methods of chemical conversion may comprise any of the non-stereospecific glutathione lyases described herein. Preferred non-stereospecific glutathione lyases include non-stereospecific glutathione lyases comprising an amino acid sequence at least about 60% identical, at least about 65% identical, at least about 70% identical, at least about 75% identical, at least about 80% identical, at least about 85% identical, at least about 90% identical, or at least about 95% identical to any of SEQ ID NO:18 (NaGST_(Nu)), residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)), SEQ ID NO:22 (SYK6GST_(Nu)), residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)), SEQ ID NO:26 (ecYghU), residues 21-313 of SEQ ID NO:28 (recombinant ecYghU), SEQ ID NO:30 (ecYfcG), SEQ ID NO:32 (ssYghU), SEQ ID NO:34 (GST3), and SEQ ID NO:36 (PcUre2pB1).

The first compound contacted with the non-stereospecific glutathione lyase in the methods of chemical conversion preferably has a structure of Formula I or a salt thereof:

wherein: R¹, R², and R³ are each independently —H, —OH, —O-alkyl, —O-lignin, or -lignin; R⁴ is —H, —OH, —SH, —COOH, —SO₃H, or —O-lignin; and SG is glutathione bound in an S or R configuration.

The second compound yielded by contacting the first compound with the non-stereospecific glutathione lyase in the methods of chemical conversion preferably has a structure of Formula II or a salt thereof:

wherein R¹, R², R³, and R⁴ are as defined above.

Contacting the first compound with the non-stereospecific glutathione lyase in the methods of chemical conversion may occur in in the presence of NADH and a GSH reductase that catalyzes reduction of glutathione disulfide, such as a GSH reductase comprising an amino acid sequence at least 95% identical to SEQ ID NO:38 (AvGR).

The first compound may be generated by contacting lignin comprising β-O-4 ether linkages with any dehydrogenase described herein and/or any β-etherase described herein. The lignin may be contacted with these enzymes in vitro.

Another aspect of the invention includes recombinant enzymes. The recombinant enzymes may be used in any of the methods described herein. The recombinant enzymes may comprise recombinant versions of any enzyme described or encompassed herein, including non-stereospecific glutathione lyases or other enzymes. The term “recombinant” used with reference to an enzyme refers to non-naturally occurring enzymes containing two or more linked polypeptide sequences. Thus, the recombinant enzymes may contain one or more non-native modifications selected from the group consisting of an amino acid addition, an amino acid deletion, and an amino acid substitution. “Non-native modification” refers to a modification that is not found in any native protein. The recombinant enzymes can be produced by recombination methods, particularly genetic engineering techniques, or can be produced by chemical synthesis.

In some versions, the recombinant enzyme comprises a recombinant non-stereospecific glutathione lyase of the invention. The recombinant non-stereospecific glutathione lyase may be a recombinant version of any of the non-stereospecific glutathione lyases described or encompassed herein. Preferred recombinant non-stereospecific glutathione lyases include non-stereospecific glutathione lyases comprising an amino acid sequence at least about 60% identical, at least about 65% identical, at least about 70% identical, at least about 75% identical, at least about 80% identical, at least about 85% identical, at least about 90% identical, or at least about 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1). The design of amino acid deletions and substitutions in the recombinant non-stereospecific glutathione lyases can be guided by the alignment of the native non-stereospecific glutathione lyases provided in FIG. 5.

Amino acid additions in the recombinant enzymes of the invention, including the recombinant non-stereospecific glutathione lyases, may comprise the addition of any amino acid on the N-terminus, the C-terminus, or both the N-terminus and C-terminus of any native enzyme.

The added amino acids may comprise fusion tags. A number of fusion tags are known in the art. Some fusion tags are used for protein detection. These include green fluorescent protein (GFP) and its many variants (Tsien 1998). Some fusion tags are used for increasing expression and solubility of proteins. These include maltose binding protein (MBP), small ubiquitin-like modifier (SUMO), and glutathione S-transferase (GST), among others (Bell et al. 2013, Butt et al. 2005). Some fusion tags, sometimes referred to as “affinity tags,” are used for purification, detection with antibodies, or other uses. A number of affinity tags are known in the art. Exemplary affinity tags include the His tag, the Strep II tag, the T7 tag, the FLAG tag, the S tag, the HA tag, the c-Myc tag, the dihydrofolate reductase (DHFR) tag, the chitin binding domain tag, the calmodulin binding domain tag, and the cellulose binding domain tag. The sequences of each of these tags are well-known in the art.

The recombinant enzymes of the invention, including the recombinant non-stereospecific glutathione lyases, may comprise a peptide cleavage sequence. The peptide cleavage sequence is preferably disposed between the enzyme portion and any fusion tag attached thereto. This permits separation of the fusion tag from the target protein, which may be useful for certain applications. The peptide cleavage sequence may be a recognition sequence for a site-specific peptidase. A number of site-specific peptidases are known in the art. These include Arg-C proteinase, Asp-N endopeptidase, Asp-N endopeptidase+N-terminal glu, BNPS-Skatole, caspase1, caspase2, caspase3, caspase4, caspase5, caspase6, caspase7, caspase8, caspase9, caspase10, chymotrypsin-high specificity (C-term to [FYW], not before P), chymotrypsin-low specificity (C-term to [FYWML], not before P), clostripain (clostridiopeptidase B), CNBr, enterokinase, factor Xa, formic acid, glutamyl endopeptidase, granzymeB, hydroxylamine, iodosobenzoic acid, Lys-C, Lys-N, NTCB (2-nitro-5-thiocyanobenzoic acid), neutrophil elastase, pepsin (pH1.3), pepsin (pH>2), proline-endopeptidase, proteinase K, SUMO proteases (Ulp1, Senp2, and SUMOstar), staphylococcal peptidase I, subtilisin BPN, tobacco etch virus (TEV) protease, thermolysin, thrombin, and trypsin, and variants thereof, among others. The cleavage recognition sites for these and other site-specific peptidases are well known in the art. Exemplary peptide cleavage sequences include the ExxYxQ↓(G/S) recognition sequence of TEV (and AcTEV and ProTEV), the LVPR↓G recognition sequence of thrombin, the IEGR↓x recognition sequence of factor Xa, and the DDDDK↓x recognition sequence of enterokinase.

The elements and method steps described herein can be used in any combination whether explicitly described or not.

All combinations of method steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise.

Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 5 to 6, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.

All patents, patent publications, and peer-reviewed publications (i.e., “references”) cited herein are expressly incorporated by reference to the same extent as if each individual reference were specifically and individually indicated as being incorporated by reference. In case of conflict between the present disclosure and the incorporated references, the present disclosure controls.

It is understood that the invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the claims.

Example 1 Nu-Class Glutathione-S-Transferases can Act as Glutathione Lyases in the Bacterial Pathway for Breaking β-Aryl Ether Bonds in Lignin Summary

As a major component of plant cells walls, lignin is a potential renewable source of valuable chemicals. Several sphingomonad bacteria have been identified that can use glutathione to break the β-aryl ether bond commonly found between phenylpropanoid units of the lignin heteropolymer. To explore bacterial strategies for depolymerizing lignin, we tested the abilities of three sphingomonads to metabolize the β-aryl ether containing dimeric aromatic compound guaiacylglycerol-β-guaiacyl ether (GGE). We found that Novo-sphingobium aromaticivorans metabolized GGE at amongst the fastest rates thus far reported. After the β-aryl ether bond of GGE is broken, the glutathione moiety must be removed from the resultant phenylpropanoid conjugate (α-glutathionyl-β-hydroxypropio-vanillone (GS-HPV)). We found that a Nu-class glutathione-S-transferase (GST) is necessary and sufficient for removing glutathione from both the R and S stereoisomers of GS-HPV in N. aromaticivorans. To investigate the prevalence of this glutathione lyase activity within related proteins, we tested Nu-class GSTs from Sphingobium sp. SYK-6 and Escherichia coli and found that they also cleave both stereoisomers of GS-HPV. We solved the crystal structure of the N. aromaticivorans Nu-class GST and used it to develop models for how this enzyme binds and cleaves both stereoisomers of GS-HPV.

Significance

There is considerable interest is identifying biological strategies to produce valuable products from renewable resources. The non-edible, lignocellulosic, fraction of plant biomass has been identified as a renewable source of bio-based products, but the properties of the aromatic polymer lignin present a major hurdle in realizing this goal. Here, we describe a novel role for a Nu-class glutathione-S-transferase (GST) in cleavage of the β-aryl ether bond that connects many aromatic units in lignin. We show that homologues of this enzyme from other bacteria also have this activity, suggesting that this function may be common throughout this widespread class of enzymes. Structural and biochemical analysis of Nu-class GSTs are used to model substrate binding and cleavage by these enzymes.

Introduction

As society looks to diversify its sources of fuels and chemicals, there are compelling reasons to develop renewable sources for them. Lignocellulosic plant biomass is the most abundant renewable material on Earth, so it is a promising but underutilized feedstock for generating products currently derived from petroleum or other non-renewable sources. Lignin, which can compose ˜25% of plant biomass (U.S. DOE 2015), is a phenylpropanoid heteropolymer containing several classes of covalent linkages, including a majority of β-aryl ether (β-O-4) bonds (Adler 1977). The properties of lignin create challenges to using it as an industrial raw material. Because of its abundance and potential value, there is interest in developing economical and environmentally sustainable methods to depolymerize lignin to its aromatic substituents. We are interested in analyzing biological processes for lignin depolymerization and conversion to valuable chemicals.

The β-etherase pathway of sphingomonadales bacteria, which cleaves the β-aryl ether bond (FIG. 1), is a promising route for lignin depolymerization. Although several species are known to contain this pathway (Masai et al. 1989, Palamuru et al. 2015, Ohta et al. 2015), characterizing the strategies and proteins employed could help to develop biological systems for depolymerizing lignin. In this work, we tested sphingomonads predicted to contain the β-etherase pathway (Ohta et al. 2015), and found that N. aromaticivorans most rapidly and completely metabolized the dimeric aromatic compound guaiacylglycerol-β-guaiacyl ether (GGE; FIG. 1). We also found that the Saro_2595 gene of N. aromaticivorans encodes a previously uncharacterized Nu-class glutathione-S-transferase (named here NaGST_(Nu)) that is capable of cleaving glutathione from both the β(R)- and β(S)-stereoisomers of the pathway intermediate β-glutathionyl-γ-hydroxypropiovanillone (GS-HPV; FIG. 1) in vitro, and we show that it is the sole enzyme in N. aromaticivorans required for this reaction in vivo.

Nu-class GSTs are found in many organisms (Stourman et al. 2011; Mashiyama et al., 2014), but their physiological roles are unknown. Although the Escherichia coli Nu-class GSTs ecYfcG and ecYghU can reduce the disulfide bond of 2-hydroxyethyl disulfide in vitro (Stourman et al. 2011, Wadington et al. 2009, Wadington et al. 2010), the physiological relevance of this reaction has not been established. To test the prevalence of the glutathione lyase activity in Nu-class GSTs, we assayed ecYghU, ecYfcG, and a Nu-class GST from Sphingobium sp. SYK-6 (encoded by SLG_04120; named here SYK6GST_(Nu)), and found that they also cleave β(R)- and β(S)-GS-HPV in vitro. Furthermore, ecYghU complements growth of an N. aromaticivorans ANaGST_(Nu) mutant, showing that it is active in vivo.

Crystal structures reported here show that NaGST_(Nu) is similar to ecYghU (Stourman et al. 2011) and Streptococcus sanguinis SK36 YghU (Patskovsky et al.), although there are notable differences in the channels leading to the active sites. We propose a mechanism for the glutathione lyase activity of the Nu-class GSTs and use molecular modeling to show how the active site channels of these enzymes can accommodate the β(R)- and β(S)-stereoisomers of GS-HPV. Indeed, the approach to the active site of these Nu-class GSTs can accommodate a variety of GSH-conjugated substrates, independent of C—S bond stereochemistry, including other GSH-conjugates derived from lignin depolymerization, or ones that might be found in organisms that do not metabolize lignin.

General Materials and Methods

Bacterial Strains and Growth Media.

Strains used are listed in Table 1. Unless otherwise noted, E. coli cultures were grown in Lysogeny Broth (LB), and shaken at ˜200 rpm at 37° C. For routine storage and manipulation, sphingomonad cultures were grown in LB at 30° C. GluSis is a modification of Sistrom's minimal medium (Sistrom 1962) in which the succinate has been replaced by 22.6 mM glucose. Standard Mineral Base (SMB) (Stanier et al. 1966) contains 20 mM Na₂HPO₄, 20 mM KH₂PO₄, 1 g/L (NH₄)₂SO₄, and 20 mL Hutner's vitamin-free concentrated base (adapted from (Cohen et al. 1957), but lacking nicotinic acid, thiamin, and biotin) per liter, final pH 6.8. SMB was supplemented with carbon sources as described below. Where needed to select for plasmids, media were supplemented with 50 μg/mL kanamycin and/or 10 μg/mL chloramphenicol.

TABLE 1 Bacterial strains and plasmids used. Strains Relevant characteristics References Novosphingobium aromaticivorans strains DSM 12444 Wild-type; also called F199 Fredrickson et al. 1991 12444Δ1879 DSM 12444 ΔSaro_1879 This study 12444Δ2595 12444Δ1879 ΔSaro_2595 This study 12444ecyghU 12444Δ2595 containing E. coli yghU at This study the Saro_2595 locus Novosphingobium sp. Wild-type Notomista et al. 2011 PP1Y Sphingobium Wild-type; also called BN6^(T) and DSM Stolz et al. 2000; Pal et xenophagum NBRC 6383^(T) al. 2006 107872 Escherichia coli strains DH5α F-ϕ80lacZ ΔM15 Δ(lacZYA-argF) Bethesda Research U169 recA1 endA1 hsdR17 (rK−, Laboratories mK+) phoA supE44 λ− thi-1 gyrA96 Taylor (1993) relA1 S17-1 recA pro hsdR RP4-2-Tc::Mu-Km::Tn7 Simon et al. 1983 Turbo F′proA⁺B⁺ lacI^(q) ΔlacZM15/ New England Biolabs fhuA2 Δ(lac-proAB) glnV galK16 galE15 R(zgb-210::Tn10)Tet^(S) endA1 thi-1 Δ(hsdS-mcrB)5 NEB 5-alpha Competent fhuA2 Δ(argF-lacZ)U169 phoA glnV44 New England Biolabs E. coli ϕ80Δ(lacZ)M15 gyrA96 recA1 relA1 endA1 thi-1 hsdR17 E. cloni 10G F- mcrA Δ(mrr-hsdRMS- Lucigen mcrBC) endA1 recA1 ϕ 80d/acZΔM15ΔlacX74 araD139 Δ(ara,leu)7697galU galK rpsL nupG λ- tonA (StrR) B834 F-hsdS metE gal ompT Wood 1966; Doherty 1995 Plasmids pK18mobsacB pMB1ori sacB kan^(R) mobT oriT(RP4) Schafer et al. 1994 lacZα pK18msB-MCS1 pK18mobsacB lacking the multiple Present example cloning site pVP302K lac promoter lacI, Tev site rtxA (V. Supplemental cholera) kan^(R); coding sequence for Materials of Gall and 8 × His-tag Ralph et al. 2014 pRARE2 p15a ori camR; tRNA genes for 7 rare Novagen codons in E. coli pK18msB/ΔSaro1879 pK18 mobsacB containing genomic Present example regions flanking Saro_1879 pK18msB/ΔSaro2595 pK18 mobsacB containing genomic Present example regions flanking Saro_2595 pK18msB/ecyghU- pK18mobsacB containing E. coli yghU Present example Δ2595 between the Saro_2595 flanking regions pVP302K/Ctag-2595 pVP302K containing Saro_2595 Present example upstream of rtxA and His-tag sequence pVP302K/Untagged2595 pVP302K containing Saro_2595 Present example pVP302K/Ntag-2595 pVP302K containing Saro_2595 Present example downstream of His-tag coding sequence and Tev protease site pVP302K/Ntag-ecyghU pVP302K containing yghU downstream Present example of His-tag coding sequence and Tev protease site pVP302K/Ntag-ecyfcG pVP302K containing yfcG downstream Present example of His-tag coding sequence and Tev protease site pVP302K/Ntag- pVP302K containing SLG_04120 Present example SLG_04120 downstream of His-tag coding sequence and Tev protease site

Construction of Defined N.

aromaticivorans mutants. We deleted Saro_1879 from the Novosphingobium aromaticivorans DSM 12444 genome to create a strain (12444Δ1879) amenable to genomic modifications using a sacB-containing vector. We used 12444Δ1879 to generate strains in which Saro_2595 was deleted from the genome (12444Δ2595) and in which Saro_2595 was replaced in the genome by the E. coli yghU gene (12444ecyghU).

Sphingomonad Growth Experiments.

Cell densities were measured using a Klett-Summerson photoelectric colorimeter with a red filter. For N. aromaticivorans, 1 Klett Unit (KU) is equal to ˜8×10⁶ cells/mL (Table 2). Experimental cultures of N. aromaticivorans and Novosphingobium sp. PP1Y were grown in SMB containing either vanillate or GGE alone (4 mM and 3 mM, respectively), or a combination of vanillate and GGE (4 mM and 1.5 mM, respectively). For S. xenophagum cultures, vanillate was replaced by glucose, since we found this strain to be unable to metabolize vanillate. N. aromaticivorans was also grown in SMB containing 200 μM GGE. Starter cultures were grown in SMB with 4 mM vanillate or glucose, and cells were pelleted and washed with PBS (10 mM Na₂HPO₄, 1.8 mM KH₂PO₄, 137 mM NaCl, 2.7 mM KCl; pH=7.4). Pellets were resuspended into culture medium and used to inoculate experimental cultures to initial cell densities of <5 KU.

Cultures were incubated at 30° C., in 125 mL conical flasks containing 20-40 mL of medium and shaken at ˜200 rpm. Aliquots (400-600 μL) were removed at specified time points and filtered through 0.22 μm syringe tip filters (e.g., Whatman Puradisc filters, GE Healthcare, Piscataway, N.J.) before HPLC analysis of extracellular aromatics. Every culture was grown at least three times; data shown are from representative cultures.

TABLE 2 Relationship between Klett Units (KU) and colony forming units (CFUs) for Novosphingobium aromaticivorans +/− sucrose (CFU mL⁻¹ KU⁻¹). −sucrose +sucrose DSM 12444 8.0 (±3.2) × 10⁶ 0 12444Δ1879 8.1 (±1.7) × 10⁶ 7.3 (±3.4) × 10⁶

To acquire these data, cultures of Novosphingobium aromaticivorans DSM 12444 and 12444Δ1879, were grown in liquid medium. Cell densities were measured using a Klett-Summerson photoelectric colorimeter with a red filter. Cultures were diluted, then plated onto solid media +/−10% sucrose.

Enzyme Purifications.

Genes Saro_2595, E. coli yghU, E. coli yfcG, and SLG_04120 from Sphingobium sp. SYK-6 (codon-optimized for expression in E. coli) were individually cloned into plasmid pVP302K (Gall and Ralph et al. 2014) so that transcripts from the plasmids would be translated into proteins containing a His₈-tag connected to the N-terminus via a tobacco etch virus (TEV) protease recognition site. Recombinant proteins were expressed in E. coli B834 (Wood 1966, Doherty et al. 1995) containing plasmid pRARE2 (Novagen, Madison, Wis.) grown for ˜24 h at 27° C. in ZYM-5052 Autoinduction Medium (Studier 2005) containing kanamycin and chloramphenicol. Recombinant proteins were purified essentially as described previously (Gall and Kim et al. 2014); see below for modifications to the procedure. After removal of His₈-tags using TEV protease, recombinant proteins retained a Ser-Ala-Ile-Ala-Gly-peptide on their N-termini, derived from the linker between the protein and the TEV protease recognition site. Recombinant LigE and LigF1 from N. aromaticivorans were purified as previously described (Gall and Ralph et al. 2014).

Kinetics Analysis of GS-HPV Cleavage.

The Reaction Buffer (RB) consisted of 25 mM Tris-HCl (pH 8.5) and 25 mM NaCl. The β(R)- and β(S)-stereoisomers of GS-HPV were separately generated by incubating racemic MPHPV (0.46 mM) in RB with 5 mM reduced glutathione (GSH) and either 38 μg/mL LigF1 or 36 μg/mL LigE, respectively, for several h. This sample, containing a single GS-HPV stereoisomer, guaiacol, and the unreacted MPHPV stereoisomer, was diluted with RB to achieve the desired concentration of GS-HPV for the time course reaction (0.005, 0.011, 0.022, 0.096, or 0.193 mM). An additional 5 mM GSH (dissolved in RB) was added prior to initiation of each time course. At time zero, 100 μL of the indicated enzyme sample (resuspended in RB) was combined with 1800 μL of the diluted GS-HPV reaction mixture to achieve final concentrations of 8 nM NaGST_(Nu), 195 nM ecYghU, 195 nM ecYfcG, or 47 nM (for β(R)-GS-HPV reactions) or 18 nM (for β(S)-GS-HPV reactions) SYK6GST_(Nu). Assays were performed at 25° C., and at specified time points, 300 μL of the reaction was removed and combined with 100 μL 1 M HCl (Acros Organics; Geel, Belgium) to stop the reaction before HPLC analysis to quantify HPV formed.

N. Aromaticivorans Crude Cell Extract Assays.

N. aromaticivorans cells were grown in 500 mL conical flasks containing 267 mL SMB medium with 4 mM vanillate and 1 mM GGE. When cell densities reached ˜1×10⁹ cells/mL, cells were lysed by the sonication procedure used to generate E. coli lysates for protein purification. Samples were centrifuged at 7000×g for 15 minutes, and the supernatants were used as crude cell extracts.

Assays containing 50 mM Tris-HCl (pH 8.0), 50 mM NaCl, 10 mM GSH, 0.407 mM racemic MPHPV (a mixture of the β(R)- and β(S)-isomers) and cellular extract from 12444Δ1879 or 12444Δ2595 (final concentrations of 269 and 186 μg protein/mL, respectively) were performed at 30° C. At defined time points, 300 μL aliquots were combined with 100 μL 1 M HCl to stop the reaction before HPLC analysis. At the indicated time, recombinant NaGST_(Nu) (30 μg protein/mL) was added to the 12444Δ2595 cell extract reaction, along with an additional 10 mM GSH.

HPLC Analysis.

After extracellular aromatics were identified using LC-MS, routine analysis and quantification of aromatics were performed using an Ultra AQ C18 5 μm column (Restek, Bellefonte, Pa.) attached to a System Gold HPLC (Beckman Coulter, Brea, Calif.) with running buffers and chromatography conditions described in FIG. 14. The eluent was analyzed for light absorbance between 191 and 600 nm, and absorbances at 280 nm were used for quantification of aromatic metabolites by comparing peak areas to those of standards (retention times of measured metabolites are shown in FIG. 15).

Production of cDNA Libraries from N. Aromaticivorans Cultures and Real-Time PCR.

N. aromaticivorans cultures were grown in 120 mL of SMB containing either 4 mM vanillate or 4 mM vanillate and 1 mM GGE. When the cultures reached densities of ˜8×10⁸ cells/mL, 40 mL were removed and combined with 5.71 mL ice-cold Stop Solution (95% ethanol, 5% acid phenol: chloroform (5:1 solution, pH 4.5)). These mixtures were centrifuged at 4° C. for 12 min at 6,000×g. Cell pellets were resuspended into 2 mL Lysis Solution (2% SDS, 16 mM EDTA in RNase-free water), then incubated at 65° C. for 5 min. RNA purification and cDNA synthesis were performed as previously described (Tavano et al. 2005), using SuperScript III reverse transcriptase (Thermo Fisher Scientific, Waltham, Mass.) to construct the cDNA library.

Real-time PCR was performed on a 7500 Real Time PCR System (Applied Biosystems, Forest City, Calif.) using SYBR Green JumpStart Taq ReadyMix (Sigma-Aldrich, St. Louis, Mo.). Primers used to detect transcripts are contained in Table 3. Transcript levels were normalized to those of Saro_0141 (rpoZ, coding for the RNA polymerase omega subunit).

TABLE 3 Primers used for RT qPCR of Novosphingobium aromaticivorans genes. Transcript assayed for Primers Saro_0141 5′-GAGATCGCGGAAGAAACCGTGC-3′ (SEQ ID NO: 48) (rpoZ) 5′-GATTTCATCCACCTCGTCGTCGTC-3′ (SEQ ID NO: 49) Saro_0205 (ligD) 5′-CAACATCAAGTCGAACATCGCGGAAG-3′ (SEQ ID NO: 50) 5′-CTGGTGGATCGAATGCAGCGAG-3′ (SEQ ID NO: 51) Saro_0793 (ligO) 5′-GATCGAGGAATCTTCCTACGACGACTG-3′ (SEQ ID NO: 52) 5′-GTTTACCACGCCGTGGAGGTTCAC-3′ (SEQ ID NO: 53) Saro_0794 (ligN) 5′-CATATCGTCTGCACCGCTTCGATGTC-3′ (SEQ ID NO: 54) 5′-GCAGAATGCCGAGCAGATCACG-3′ (SEQ ID NO: 55) Saro_1875 (ligL) 5′-CCATGTCGTCAACACCGCATCG-3′ (SEQ ID NO: 56) 5′-CATGTTCTCGGTCAGGTTCAGCAC-3′ (SEQ ID NO: 57) Saro_2091 (ligF) 5′-GCTGCTGACGGTGTTCGAGAAG-3′ (SEQ ID NO: 58) 5′-CTTGAACCAGTCGGTGTGATGCTC-3′ (SEQ ID NO: 59) Saro_2405 (ligP) 5′-CATCGTCGAATACCTCGATGCCAAGTATC-3′ (SEQ ID NO: 60) 5′-GTCCTGGCAGAAGCAGAACATCCAC-3′ (SEQ ID NO: 61) Saro_2595 5′-CCACGATCATGCTGGAAGAACTGCTC-3′ (SEQ ID NO: 62) 5′-GATTCGAAGACGCGGAACGGTTCAG-3′ (SEQ ID NO: 63)

Determination of Chemical Oxygen Demand (COD).

Initial culture COD values were obtained either from uninoculated medium, or from inoculated medium that was immediately passed through a 0.22 μm filter. Final COD values were obtained when cultures reached their maximum cell densities. For these samples, we analyzed the COD of both the entire culture (cells and medium) and the filtered medium. The difference in COD between the unfiltered and filtered final samples is defined as the COD of cellular biomass. Samples were diluted as needed and combined with High Range COD Digestion Solution (Hach, Loveland, Colo.). The mixtures were heated to 150° C. for 120 min to oxidize the materials before absorbances were measured at 600 nm. Standards with known COD values were analyzed in parallel.

Chemicals.

Vanillate, guaiacol, reduced L-glutathione (GSH), and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) were purchased from Sigma-Aldrich (St. Louis, Mo.). erythro-Guaiacylglycerol-β-guaiacyl ether (erythro-GGE) was purchased from TCI America (Portland, Oreg.).

A racemic mixture of β-(2-methoxyphenoxy)-γ-hydroxypropiovanillone (MPHPV) was synthesized by dissolving erythro-GGE into ethyl acetate (Fisher Scientific), then slowly adding 1.25 molar equivalents of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and stirring for 30 min. The reaction was washed three times with saturated NaHCO₃ to remove DDQH₂ formed during the reaction. The MPHPV was purified via flash chromatography using hexane/ethyl acetate=1/3, as previously described (Gall and Kim et al. 2014), then crystallized from the eluent via solvent evaporation.

Hydroxypropiovanillone (HPV) was synthesized as previously described for synthesis of β-deoxy-α-veratrylglycerone, except using 4-O-benzyl-acetovanillone as starting material, rather than acetoveratrone (Gall and Kim et al. 2014). Synthesis of HPV required an additional debenzylation step that was unnecessary in the synthesis of β-deoxy-α-veratrylglycerone.

Structural Analysis of NaGST_(Nu) and Molecular Modeling.

NaGST_(Nu) was screened for crystal formation against several commercial screens at 277 and 293K using a TTP Labtech Mosquito® crystallization robot. The best diffracting ammonium acetate precipitated crystal was obtained at 293K, by mixing 0.2 μL of protein solution (277 μM protein preincubated for 50 min with 10 mM GSH (neutralized with NaOH)) with 0.2 μL of reservoir solution (4 M ammonium acetate buffered with 100 mM sodium acetate, pH 4.6). This crystal was mounted directly from the growth solution by drawing it through a layer of fomblin oil, thinning the surrounding liquid with a paper wick, and was plunged into liquid nitrogen. The best diffracting ammonium sulfate precipitated crystal was obtained at 293K using 0.13 μL of protein solution and 85 nL of reservoir solution (1.35 M ammonium sulfate, 0.1 M lithium sulfate, and 0.1 M bis-trispropane, pH 7.5). This crystal was cryopreserved by adding 0.5 μL of a solution composed of 2 parts reservoir solution and one part neat glycerol to the droplet containing the crystal, and equilibrating for 11 min prior to looping and plunging into liquid nitrogen.

Diffraction data were obtained at the GM/CA beam-line at Argonne National Laboratory with an Eiger 16M detector (Casanas et al. 2016). Data were collected on the ammonium acetate and ammonium sulfate crystal forms using 1.033 Å (for 1.45 Å resolution) or 0.7749 Å (for 1.25 Å resolution) X-rays, respectively. Diffraction data were reduced using XDS (Kabsch 2010). Both crystals belonged to space group P2₁2₁2₁ with a predicted solvent content of 60%. The structure was solved by molecular replacement with Phaser (McCoy et al. 2007) in the Phenix suite (Adams et al. 2010), using a search model based on PDB ID 3C8E:A (ecYghU (Stourman et al. 2011)) modified with phenix.sculptor (Bunkóczi et al. 2011), based on primary sequence alignment. Structure solution revealed two copies of the protein per asymmetric unit, with strong electron density present for the paired active site glutathione molecules. Phenix.refine (Afonine et al. 2012) and COOT (Emsley et al. 2004) were alternatively used to refine the structure and fit the model to electron density maps.

For modeling β(R)- and β(S)-GS-HPV and their syringyl phenylpropanoid analogues into the active sites of NaGST_(Nu) and ecYghU, the PyMOL Builder function (The PyMOL Molecular Graphics System, Version 1.8.2.1) was used to create molecules of GS-HPV or GS-syringyl by adding atoms onto the GSH2 molecule bound in each active site. Atoms were added so as to visually minimize steric clash with the proteins. The potential energies of the protein-GS-phenylpropanoid complexes were minimized using the Minimize Structure function of UCSF Chimera (Pettersen et al. 2004). For the energy minimization, all of the atoms of the protein-GS-phenylpropanoid complex were held rigid except for those of the phenylpropanoid moiety and the Cys sidechain of GSH2. One-hundred steepest descent steps were run, followed by twenty conjugate gradient steps, and all step sizes were 0.05 Å.

Recipes for Media Components:

-   -   Hutner's vitamin-free Concentrated Base (Cohen-Bazire et         al. 1957) (per 500 mL):         -   5 g Nitrilotriacetic acid         -   14.78 g MgSO₄.7H₂O         -   1.67 g CaCl₂.2H₂O         -   4.625 mg (NH₄)₆Mo₇O₂₄.4H₂O         -   49.5 mg FeSO₄.7H₂O         -   25 mL Metals “44”     -   Metals “44” (per 500 mL):         -   1.25 g EDTA (free acid) [Add this first; use 10 M NaOH to             help dissolve it.]         -   5.475 g ZnSO₄.7H₂O         -   2.5 g FeSO₄.7H₂O         -   0.77 g MnSO₄.H₂O         -   0.196 g CuSO₄.5H₂O         -   0.125 g Co(NO₃)₂.6H₂O         -   0.0885 g Na₂B₄O₇.10H₂O

Construction of Defined N. aromaticivorans Mutants.

N. aromaticivorans has been reported to use sucrose as sole carbon source (Stolz et al. 2000), but we found it to be incapable of growing in the presence of ≥10% sucrose (Table 2). We also noticed that the gene Saro_1879 is annotated as sacB, whose product, levansucrase, makes sucrose inhibitory to growth of many Gram-negative bacteria (Gay et al. 1985). To create an N. aromaticivorans strain that we could modify using a sacB-containing plasmid, we deleted Saro_1879 from its genome. To do this, we constructed plasmid pK18msB/ΔSaro1879, in which genomic DNA from upstream and downstream of Saro_1879 was cloned into pK18msB-MCS1 (for details of plasmid construction, see below). This plasmid was mobilized into N. aromaticivorans via electroporation using a single 2.5 kV pulse in a 0.2 cm cuvette in a MicroPulser apparatus (Bio-Rad, Hercules, Calif.). N. aromaticivorans was made electrocompetent by washing exponential phase cells from LB cultures twice with ice-cold 0.5 M glucose, then resuspending the cells into 10% glycerol. Transformants in which the plasmid was integrated into the genome via homologous recombination (single crossovers) were selected for by growth on solid LB containing kanamycin. These strains were grown in liquid LB to allow plasmid loss via homologous recombination, then plated on solid LB containing 10% sucrose to select for sucrose-tolerance. Sucrose-tolerant strains in which Saro_1879 had been deleted from the genome were confirmed by PCR amplification and sequencing of genomic DNA. One of these strains (12444Δ1879) was used as the parent strain for subsequent genetic modifications.

To inactivate Saro_2595, we electroporated plasmid pK18msB/ΔSaro2595 (in which genomic DNA from upstream and downstream of Saro_2595 was cloned into pK18msB-MCS1) into strain 12444Δ1879, and a strain lacking the Saro_2595 gene (referred to as 12444Δ2595) was isolated via the process described above for deleting Saro_1879.

To generate a strain (referred to as 12444ecyghU) in which Saro_2595 was replaced in the N. aromaticivorans genome with the E. coli yghU gene, we constructed plasmid pK18msB/ecyghU-42595, in which yghU (amplified from E. coli DH5α genomic DNA) was cloned into pK18msB/ΔSaro2595. This plasmid was mobilized into 12444Δ2595 via conjugation from E. coli S17-1. Transconjugants (single crossovers) of N. aromaticivorans were isolated on solid GluSis containing kanamycin. After growth in liquid GluSis, strains that lost the plasmid and retained the yghU gene at the native Saro_2595 locus were isolated on solid GluSis medium containing sucrose. The presence of the yghU gene in the genome was confirmed via PCR and sequencing.

Purification of Recombinant Proteins.

Recombinant proteins were purified as described previously (Gall and Kim et al. 2014), except that cells were lysed by sonication, first using a Branson Sonifier 450 (Branson Ultrasonics, Danbury, Conn.) (duty cycle 50%, output level 6, for six rounds of 1-2 min), then a Qsonica Q500 with a cup horn attachment (Qsonica, Newtown, Conn.) (60% amplitude for 10 minutes with cycles of 10 s on, 10 s off). His-tagged proteins were purified from lysates over a column packed with Ni²⁺-NTA resin (Qiagen, Hilden, Germany) attached to an AKTAprime plus FPLC (GE Healthcare Life Sciences), then incubated with TEV protease to remove the His-tag. The protease reaction mixture was passed through a Ni²⁺-NTA column to separate the recombinant protein from the cleaved His₈-tag and TEV protease (which also contained a His-tag).

Procedures to Construct Plasmids for Genomic Modifications for N. Aromaticivorans and for Purifying Enzymes

Biological reagents. All PCR reactions were performed with Herculase II polymerase (Agilent Technologies, Santa Clara, Calif.). Primers were phosphorylated with polynucleotide kinase from Promega (Madison, Wis.). All other enzymes were from New England Biolabs (Ipswich, Mass.). All primers were from Integrated DNA Technologies (Coralville, Iowa). See Table 4.

TABLE 4 Primers used in genomic modifications and enzyme expression. Name Sequence Notes pK18msB AseI 5′-CTGTCGTGCCAGCTGCATTAATG-3′ (SEQ AseI site ampl F ID NO: 64) (underlined) native to template pK18msB- 5′-GAACAtcTAGAAAGCCAGTCCGCAGAA XbaI site MCS XbaI R AC-3′ (SEQ ID NO: 65) (underlined); lowercase bases do not match template Saro1879 5′-CCCGAattaATCGTGACGGTATCAACCT AseI site lvnsucr ampl F CC-3′ (SEQ ID NO: 66) (underlined); AseI lowercase bases do not match template Saro1879 5′-GTTTCGGtCtAGATCGAGCTGACCGAA XbaI site lvnsucr ampl R ATC-3′ (SEQ ID NO: 67) (underlined); XbaI lowercase bases do not match template Saro_2595 5′-GTCGatTAatAGTCCGAGATCGAGGC AseI site amp AseI for TGC-3′ (SEQ ID NO: 68) (underlined); lowercase bases do not match template Saro_2595 5′-CGACtctAGaCAGAGCCTGAACGAA XbaI site amp XbaI rev GTC-3′ (SEQ ID NO: 69) (underlined); lowercase bases do not match template Saro1879 5′-CCGACTTTCTTGAAACAGATTTGGCTT lvnsucr del AAGAC-3′ (SEQ ID NO: 70) REV Saro1879 5′-GTTCATGCTTAACTTCGATGGCGAGC-3′ lvnsucr del (SEQ ID NO: 71) FOR Saro_2595 del 5′-CCTGCTCCTTGGGGATATTGTTAGTG rev TTG-3′ (SEQ ID NO: 72) Saro_2595 del 5′-GGAATCGTTGCAAGCGATCGTCAAG-3′ for (SEQ ID NO: 73) D2595pK18- 5′-GGAGCAGGCGATGACAGACAATACTT underlined region ecYghU F ATCAGCCCGCGAAAG-3′ (SEQ ID NO: 74) matches sequence in pK18msB/ΔSaro2595 D2595pK18- 5′-CGAGGCGGGTTTACCCCTGACGCTTAT underlined region ecYghU R CTTCCGTATTCGTC-3′ (SEQ ID NO: 75) matches sequence in pK18msB/ΔSaro2595 ecYghU- 5′-TCAGGGGTAAACCCGCCTCGAGACCG underlined region D2595pK18 F GCGAAC-3′ (SEQ ID NO: 76) matches sequence in yghU ecYghU- 5′-TGTCTGTCATcgCCTGCTCCTTGGGGAT underlined region D2595pK18 R ATTGTTAGTG-3′ (SEQ ID NO: 77) matches sequence in yghU; lowercase bases are not present in pK18msB/ΔSaro2595, but are present in the N. aromaticivorans genome Saro2595 Ctag 5′-GCAGGacATGTCCTCAGAGTACGTTCC-3′ PciI site PciI F (SEQ ID NO: 78) (underlined); lowercase bases do not match template Saro2595 Ctag 5′-GTTatctgcgagaccACGATCGCTTGCAACG BsaI site BsaI R ATTC-3′ (SEQ ID NO: 79) (underlined); lowercase bases do not match template pVP302K Ctag 5′-CTGCGGTCTCGCAGATGGTAAAATT BsaI site BsaI F CTG-3′ (SEQ ID NO: 80) (underlined) pVP302K Ctag 5′-GGTGATGTCCCATGGTTAATTTCTCCTC NcoI site NcoI R TTTAATG-3′ (SEQ ID NO: 81) (underlined) Ctag 2595-pVP 5′-tcagaagcccttgACGATCGCTTGCAACGA lowercase bases do add Stop R TTC-3′ (SEQ ID NO: 82) not match pCtag- 2595/pVP302K; underlined bases are stop codon pVP302K Ntag 5′-CATTAAaAGcTTAAACGAATTCGGACT HindIII site HindIII F CGGTACGC-3′ (SEQ ID NO: 83) (underlined); lowercase bases do not match template 2595-pVP C to 5′-caagcgaaaatctgtattttcagagcgcgatcgcaggaATG lowercase bases do Ntag F TCCTCAGAGTACGTTCC-3′ (SEQ ID NO: 84) not match template; bold ATG is Saro_2595 start site; underlined region is Tev protease recognition site pVP302 C to 5′-ccaatgcatggtgatggtgatgatggtgatgtcccatGGTT lowercase bases do Ntag R AATTTCTCCTCTTTAATG-3′ (SEQ ID NO: 85) not match template; compliment of coding region for 8X-Histidine tag is underlined pVP302K- 5′-GATCGCAGGAATGACAGACAATACTT underlined region ecYghU F ATCAGCCCGCGAAAG-3′ (SEQ ID NO: 86) matches sequence in pVP302K pVP302K- 5′-CGGCTTTCTGTTACCCCTGACGCTTATC underlined region ecYghU R TTCCGTATTCGTC-3′ (SEQ ID NO: 87) matches sequence in pVP302K ecYghU- 5′-TCAGGGGTAACAGAAAGCCGAAAATA underlined region pVP302K F ACAAAGTTAGCCTGAGCTG-3′ (SEQ ID matches sequence in NO: 88) yghU ecYghU- 5′-TGTCTGTCATTCCTGCGATCGCGCTCT underlined region pVP302K R GAAAATACAGATTTTCG-3′ (SEQ ID NO: 89) matches sequence in yghU pVP302K- 5′-TCCTGCGATCGCGCTCTGAAAATACAG HiFi-ATW-R ATTTTCG-3′ (SEQ ID NO: 90) pVP302K- 5′-CAGAAAGCCGAAAATAACAAAGTTAG HiFi-ATW-F CCTGAGCTG-3′ (SEQ ID NO: 91) ecYfcG-pVP- 5′- lowercase region Ntag-HiFi-F gtattttcagagcgcgatcgcaggaATGATCGATCTCTA complementary to TTTCGCCCCGACAC-3′ (SEQ ID NO: 92) “pVP302K-HiFi- ATW-R” ecYfcG-pVP- 5′- lowercase region Ntag-HiFi-R ctaactttgttattttcggctttctgTTAACTATCCGAACGC complementary to TCATCACCGAGTTG-3′ (SEQ ID NO: 93) “pVP302K-HiFi- ATW-F” SYK6 yghU 5′-gttattttcggctttctgttaagCTTCGGTCTTCG-3′ Lowercase region pVP fix R (SEQ ID NO: 94) complementary to “SYK6 yghU pVP fix F” SYK6 yghU 5′-cttaacagaaagccgaaaataacAAAGTTAGCCT Lowercase region pVP fix F GAG-3′ (SEQ ID NO: 95) complementary to “SYK6 yghU pVP fix R”

Construction of pK18msB-MCS1.

Plasmid pK18mobsacB (Schafer et al. 1994) was amplified via PCR with phosphorylated primers “pK18msB AseI ampl F” and “pK18msB-MCS XbaI R”. The product was circularized with T4 DNA ligase, then transformed into E. coli DH5α. The final 5278-base pair (bp) plasmid, pK18msB-MCS1, is similar to pK18mobsacB, except that the multiple cloning site has been removed, and a new XbaI site is 24 bp from one of the plasmid's native AseI sites (with the other native AseI site removed).

Plasmids for Deleting Saro_1879 or Saro_2595.

Regions of N. aromaticivorans genomic DNA containing either Saro_1879 or Saro_2595, along with ˜1300-1600 bp flanking regions upstream and downstream of each gene, were separately amplified via PCR using the primer pairs “Saro1879 lvnsucr ampl F AseI”/“Saro1879 lvnsucr ampl R XbaI” and “Saro_2595 amp AseI for”/“Saro_2595 amp XbaI rev”. The amplified DNA fragments were digested with AseI and XbaI, then ligated with T4 DNA ligase into AseI- and XbaI-digested pK18msB-MCS1. The resulting plasmids (pK18msB/Saro1879 and pK18msB/Saro2595) were transformed into E. coli (strain DH5α for the Saro_1879 plasmid and Turbo for the Saro_2595 plasmid). PCR was performed on the purified plasmids using the phosphorylated primer pairs “Saro1879 lvnsucr del REV”/“Saro1879 lvnsucr del FOR” or “Saro_2595 del rev”/“Saro_2595 del for” to generate linear plasmids lacking the majority of the Saro_1879 and Saro_2595 coding regions, respectively. These DNA fragments were circularized with T4 DNA ligase to form plasmids pK18msB/ΔSaro1879 and pK18msB/ΔSaro2595.

Plasmid for incorporating E. coli yghU into the N. aromaticivorans genome. The yghU gene (locus tag Ga0077588_1407) was amplified from E. coli DH5α genomic DNA using primers “D2595pK18-ecYghU F” and “D2595pK18-ecYghU R”, which each contain a sequence on their 5′ end that is complementary to a region in the plasmid pK18msB/ΔSaro2595. pK18msB/ΔSaro2595 was amplified with the primers “ecYghU-D2595pK18 F” and “ecYghU-D2595pK18 R”, which each contain a sequence on their 5′ ends that is complementary to E. coli yghU, to generate a linear fragment in which pK18msB-MCS1 contains the regions flanking Saro_2595 in the N. aromaticivorans genome, along with the regions complementary to yghU.

The PCR amplified fragments were connected using NEBuilder HiFi DNA Assembly Master Mix (New England Biolabs) (using 81 ng of the linear pK18msB/ΔSaro2595 fragment and 22 ng of the yghU fragment) and transformed into NEB 5-alpha competent E. coli (New England Biolabs). The resulting plasmid (pK18msB/ecyghU-Δ2595) consisted of pK18msB-MCS1 containing the E. coli yghU gene flanked by regions that flank Saro_2595 in the N. aromaticivorans genome (i.e., with the start and stop codons positioned where the respective codons of Saro_2595 would naturally be).

Enzyme Expression Plasmids

Recombinant Saro_2595. Saro_2595 was amplified from N. aromaticivorans genomic DNA with the primers “Saro2595 Ctag PciI F” and “Saro2595 Ctag BsaI R”. This fragment was digested with PciI and BsaI. The expression vector pVP302K (Gall and Ralph et al. 2014) was amplified using the primers “pVP302K Ctag BsaI F” and “pVP302K Ctag NcoI R”. This fragment was digested with BsaI and NcoI. The digested fragments were ligated using T4 DNA ligase, generating plasmid pVP302K/Ctag-2595, which consists of a T5 promoter followed by the coding sequences of Saro_2595, the RtxA protease from Vibrio cholerae, and an 8X-Histidine tag.

pVP302K/Ctag-2595 was amplified using phosphorylated primers “Ctag 2595-pVP add Stop R” and “pVP302K Ntag HindIII F”. This fragment was circularized using T4 DNA ligase to generate plasmid pVP302K/Untagged2595, in which a stop codon has been introduced directly after Saro_2595. pVP302K/Untagged2595 was amplified via PCR using phosphorylated primers “2595-pVP C to Ntag F” and “pVP302 C to Ntag R”. This fragment was circularized using T4 DNA ligase to generate plasmid pVP302K/Ntag-2595, which contains a T5 promoter followed by coding sequences for a His₈ tag, a tobacco etch virus (Tev) protease recognition site and Saro_2595.

Recombinant E. coli YghU.

The yghU gene was amplified from E. coli DH5a genomic DNA using the primers “pVP302K-ecYghU F” and “pVP302K-ecYghU R”. pVP302K was amplified by PCR using the primers “ecYghU-pVP302K F” and “ecYghU-pVP302K R”. The two amplified fragments, with ends that are complementary to each other, were concurrently transformed (94 ng linear pVP302K, 168 ng yghU gene, in 4 μL TE buffer) into E. cloni 10G chemically competent cells (Lucigen, Middleton, Wis.). The fragments were combined via homologous recombination in vivo (Bubeck et al. 1993), and the resulting plasmid, pVP302K/Ntag-ecyghU, was purified from the cells and verified via Sanger sequencing.

Recombinant E. coli YfcG.

The yfcG gene was amplified from E. coli DH5a genomic DNA using the primers “ecYfcG-pVP-Ntag-HiFi-F” and “ecYfcG-pVP-Ntag-HiFi-R”. pVP302K was amplified via PCR using the primers “pVP302K-HiFi-ATW-R” and “pVP302K-HiFi-ATW-F”. The two amplified fragments, with ends that are complementary to each other, were combined using NEBuilder HiFi DNA Assembly Master Mix (New England Biolabs) (100 ng linear pVP302K, 48 ng yghU gene) and transformed into NEB 5-alpha competent E. coli (New England Biolabs). The resulting plasmid, pVP302K/Ntag-ecyfcG, was purified and verified via sequencing.

Recombinant SLG_04120.

A fragment containing the SLG_04120 gene, codon-optimized for E. coli, with ends complementary to pVP302K, was ordered as a gBlock from New England Biolabs. The sequence of the fragment was:

(SEQ ID NO: 96) 5′-GTATTTTCAGAGCGCGATCGCAGGAATGGCCGACTCAGATCCATCCA TGAATCAGCCGACGGGTTACGTCCCGCCGAAAGTTTGGACCTGGGACAAA GAGAACGGCGGTCAGTTCAGCAATATCAACGCCCCTACGGCTGGTGCGCG CCAGGACGTCACGCTCCCTGTAGGGGAGCACCCTATCCAATTATATAGTC TCGGCACTCCGAATGGTCAGAAAGTTACTATCATGTTGGAAGAACTGCTG GCTGCTGGCTTTGATGCTGAGTATGACGCCTGGCTCATCAAAATCTACAC AGGCGAGCAATTCGGATCTGATTTCGTCGCCATTAACCCTAATAGCAAAA TTCCGGCTATGATGGACCATGGTCTCGATCCGCCGCTCCGTTTATTTGAG TCTGGTTCTATGTTAGTTTATCTGGCCGAAAAGTTTGGCGCATTCCTCCC GACCGAAATCCGCAAACGTACGGAAACCTTTAACTGGCTCATGTGGCAGA TGGGTTCTGCTCCTTTTGTGGGTGGTGGCTTTGGCCACTTCTATGCGTAC GCCCCATTTAAAATCGAATATGCCATTGATCGTTACGCGATGGAAACCAA GCGCCAACTGGACGTTCTGGATAAAAATCTGGCCGATCGTGAATTTATGA TCGGCGATGAAATCACCATCGCAGATTTTGCGATTTTCCCTTGGTACGGC TCGATTATGCGTGGCGGTTACAACGCGCAAGAATTCTTGAGCACTCACGA GTACCGTAACGTTGATCGCTGGGTTACGCAGCTTTCTGAACGTACGGGCG TAAAGCGTGGTCTCCTTGTCAATTCCGCGGGTCGCCCGGGAGGTGGCATT GCGGAACGCCATAGCGCGGCTGATTTAGACGCGTCGATTAAAGCGGCTGA ACAAGAGGCCGCGAAGACCGAAGCTtAACAGAAAGCCGAAAATAACAAAG TTAG-3′ (underlined regions match sequences in pVP302K)

pVP302K was amplified via PCR using the primers “pVP302K-HiFi-ATW-R” and “pVP302K-HiFi-ATW-F”.

The amplified pVP302K fragment was combined with SLG_04120 gBlock using ligation-free cloning: fragments were mixed (57 ng linear pVP302K, 84 ng SLG_04120, in 4 μL TE buffer), then transformed into E. cloni 10G competent cells (Lucigen, Middleton, Wis.). DNA sequencing was used to identify a plasmid containing the correct DNA sequence for SLG_04120 in pVP302K, resulting in plasmid pVP302K/Ntag-SLG_04120.

Identification and Quantification of Extracellular Metabolites

Initial Identification Using LC-MS.

Compounds were separated on a Phenomenex PFP 250×4.6-mm column attached to an Accela LC pump equipped with a PDA UV detector. Running buffers A (5 mM formic acid and 5% acetonitrile in H₂O) and B (methanol) were initially at 82.5% and 17.5%, respectively. Buffer B was held at 17.5% for 18 minutes, then increased to 50% over 5 minutes, held at 50% for 3 minutes, then returned to initial conditions and re-equilibrated for 4 minutes. Flow rate was 1 mL per minute. UV absorbance data from 190-500 nm at 5 Hz and single wavelength data at 254 nm (20 Hz, 9 nm bandwidth) were collected.

Samples were analyzed by high resolution, tandem mass spectrometry using a Thermo Scientific Q Exactive Orbitrap mass spectrometer. The mass spectrometer was operated in fast polarity switching mode with acquisition of MS/MS spectra of the two most abundant precursor ions from the preceding MS1 scan (50-750 Th). Resolution was 35,000 at 200 Th for MS1 scans and 17,500 at 200 Th for MS/MS scans. Capillary voltage was set at 4000V in both polarities, sheath gas at 50 units, auxiliary gas at 20 units, probe heater at 350° C., inlet capillary at 325° C., and the S-Lens at 50 units. AGC target was 1e6 for MS1 scans and 2e5 for MS/MS scans with a maximum injection time of 50 ms. The isolation width for MS/MS scans was set to 2 Th and a 5 s dynamic exclusion time was used.

Elemental compositions of the metabolites were derived from the mass measurements. From the MS/MS fragmentation patterns and previous data, we provisionally identified the metabolites. We used standards to confirm these putative identifications by matching retention times and mass spectra.

Results

GGE Metabolism by Sphingomonads.

Genomic sequences predict (Ohta et al. 2015) that N. aromaticivorans DSM 12444 (Fredrickson et al. 1991, Fredrickson et al. 1995), Novosphingobium sp. PP1Y (Notomista et al. 2011), and Sphingobium xenophagum NBRC 107872 (Stolz et al. 2000) contain genes that encode enzymes required to metabolize guaiacylglycerol-β-guaiacyl (GGE) via the bacterial β-etherase pathway (FIG. 1). To test this, we fed these bacteria erythro-GGE (FIG. 1), either as sole carbon source or in the presence of another organic molecule.

We found that N. aromaticivorans 12444Δ1879 metabolized erythro-GGE and assimilated it into cell material when fed GGE alone (FIG. 2A (panels A,B) and FIG. 3) or GGE plus vanillate (FIG. 2B (panels G,H) and Table 5). β-Etherase pathway intermediates threo-GGE, MPHPV, and HPV transiently appeared in the media of both cultures (FIGS. 2A and 2B (panels B,H) and FIG. 3), whereas the pathway intermediate guaiacol was only detected in the medium of the culture fed GGE plus vanillate (FIG. 2B (panel H)). The predicted pathway intermediate GS-HPV was not detected in the medium of either culture.

Compared to N. aromaticivorans, Novosphingobium sp. PP1Y grew significantly slower in medium containing only GGE (FIG. 4A (panels A,B)). The maximum cell density and amount of COD incorporated into biomass were the same when Novosphingobium sp. PP1Y was fed either GGE plus vanillate or vanillate only (FIG. 4A (panels E,F) and Table 5), suggesting that this strain did not convert a significant amount of GGE into biomass in the presence of vanillate. S. xenophagum did not assimilate GGE into cell material in any culture tested (based on cell density and COD measurements; FIG. 4B (panels C,G) and Table 5), although low levels of some β-etherase pathway intermediates were observed in its culture media (FIG. 4B (panels D,H)).

TABLE 5 Chemical oxygen demand (COD) analysis of bacterial cultures.^(a) % COD % COD lost from Initial Final COD Final COD incorporated the Strain Carbon souces COD^(b) (biomass)^(c) (soluble)^(d) into biomass^(e) cultue^(f) N. aromaticivorans 3 mM GGE 2100 ± 100 480 ± 70 500 ± 100 22% 53% 12444Δ1879 (FIG. 2A (A, B)) 4 mM vanillate (FIG. 1300 ± 100 530 ± 40 280 ± 80  41% 37% 2B (G)) 4 mM vanillate, 2170 ± 80  720 ± 80 340 ± 60  33% 51% 15 mM GGE (FIG. 2B (G, H)) Novophingobium sp. 4 mM vanillate (FIG. 1200 ± 100 420 ± 20 240 ± 30  36% 44% PP1Y 4A (E)) 4 mM vanillate, 2070 ± 90  420 ± 20 1080 ± 20  20% 28% 15 mM GGE (FIG. 4A (E, F) S. xenophagum NBRC 4 mM glucose 1040 ± 30  490 ± 30 340 ± 20  47% 20% 107872 (FIG. 4B (G)) 4 mM glucose, 1970 ± 50  460 ± 30 1270 ± 30  23% 12% 15 mM GGE (FIG. 4B (G, H)) N. aromaticivorans 3 mM GGE 2090 ± 40   30 ± 100 1900 ± 30   1%  6% 12444Δ2595 (FIG. 2A (C, D)) 4 mM vanillate (FIG. 1300 ± 200  550 ± 100 330 ± 70  41% 34% 2B (I)) 4 mM vanillate, 2200 ± 100 520 ± 80 1200 ± 100  23% 24% 15 mM GGE (FIG. 2B (I, J)) N. aromaticivorans 3 mM GGE 2200 ± 90   500 ± 100 500 ± 100 23% 52% 12444ecyghU (FIG. 2A (E) 4 mM vanillate (FIG. 1400 ± 300 570 ± 50 430 ± 50  42% 26% 2B (K)) 4 mM vanillate, 2400 ± 100  600 ± 100 450 ± 80  26% 55% 15 mM GGE (FIG 2B (K, L)) ^(a)Units of COD are mg/L ^(b)Initial COD is that of the medium before inoculation. ^(c)Final COD (biomass) is the difference between the unfiltered and filtered final samples. ^(d)Final COD (soluble) is the COD remaining in the medium after filtering the final sample. ^(e)% COD incorporated into biomass is the ratio of Final COD (biomass) to Initial COD. ^(f)% COD lost = 1 − (Final COD (biomass) + Final COD (soluble))/Initial COD. It is assumed that the lost COD represents the electrons in the system that were combined with oxygen during cell growth.

Transcripts of Predicted β-Etherase Pathway Genes Increase in Abundance when N. aromaticivorans Grows in the Presence of GGE.

Since N. aromaticivorans metabolized GGE, we investigated the expression of genes predicted to be involved in the β-etherase pathway in this organism. With the exception of ligL, transcription levels of the genes we tested were increased in cells grown in the presence of GGE versus its absence (Table 6). Among the GGE-induced transcripts was one derived from Saro_2595, which encodes a Nu-class glutathione-S-transferase (named here NaGST_(Nu)).

TABLE 6 Fold-changes of transcript levels in N. aromaticivorans cultures grown in vanillate with or without GGE.^(a) Gene Homologue^(b) Fold-change^(c) Saro_0205 SLG_08640, LigD (77.6%) 11 ± 3  Saro_0793 SLG_35880, LigO (41.5%) 6 ± 2 Saro_0794 SLG_35900, LigN (44.7%) 9 ± 1 Saro_1875 SLG_33660, LigL (48.7%) 1 ± 1 Saro_2091 SLG_08650, LigF (59.1%) 8 ± 1 Saro_2405 SLG_32600, LigP (65.9%) 17 ± 9  Saro_2595 MBENS4_2527, GST3 (37.6%) 8 ± 2 ^(a)Transcript levels for each culture were normalized to those of Saro_0141 (rpoZ). ^(b)Homologue is the gene that codes for a product from Sphingobium sp. SYK-6 (SLG) or Novosphingobium sp. MBES04 (MBENS4) with the highest % amino acid identity to the product of the indicated N. aromaticivorans (Saro) gene. ^(c)Fold-change is the ratio of the normalized transcript level in cells grown in the presence of GGE to that in cells grown in the absence of GGE.

NaGST_(Nu) cleaves β(R)- and β(S)-GS-HPV.

NaGST_(Nu) is 38% identical in amino acid sequence to Novosphingobium sp. MBESO4 GST3 (FIG. 5), which can cleave both β(R)- and β(S)-GS-HPV into HPV in vitro (FIG. 1; (Ohta et al. 2015)). Since N. aromaticivorans lacks any homologues of LigG (the enzyme from Sphingobium sp. SYK-6 that cleaves the β(R)-stereoisomer of GS-HPV (FIG. 1; (Masai et al. 2003)), we tested whether NaGST_(Nu) could cleave both β(R)- and β(S)-GS-HPV in N. aromaticivorans. We found that recombinant NaGST_(Nu) cleaved both β(R)- and β(S)-GS-HPV in vitro (FIG. 6). Kinetic analysis of NaGST_(Nu) showed that it had slightly higher k_(cat) and ˜5-fold higher K_(M) with β(R)-GS-HPV than with β(S)-GS-HPV, resulting in a ˜4-fold higher k_(cat)/K_(M) with the β(S)-isomer (Table 7).

TABLE 7 Kinetic parameters for the enzymatic conversion of GS-HPV into HPV.^(a) GS- k_(cat) K_(M) k_(cat)/K_(M) Protein HPV^(b) (s⁻¹) (μM) (mM⁻¹s⁻¹) NaGST_(Nu) (8 nM) β(R) 80 ± 10 40 ± 6  1900 ± 400 SYK6GST_(Nu) (47 nM) β(R) 13 ± 1  55 ± 7  240 ± 40 ecYghU (195 nM) β(R) 0.43 ± 0.03 28 ± 4  16 ± 3 ecYfcG (195 nM) β(R) 0.04 ± 0.01 160 ± 60   0.2 ± 0.1 NaGST_(Nu) (8 nM) β(S) 57 ± 9  8 ± 3  8000 ± 3000 SYK6GST_(Nu) (18 nM) β(S) 30 ± 5  11 ± 2  2700 ± 700 ecYghU (195 nM) β(S) 0.29 ± 0.03 12 ± 3  24 ± 6 ecYfcG (195 nM) β(S) 0.017 ± 0.004 130 ± 40   0.14 ± 0.06 ^(a)Kinetic data are from fits shown in FIG. 6 ^(b)Stereoisomer of GS-HPV used in reaction (see FIG. 1)

NaGST_(Nu) is Necessary for GGE Metabolism In Vivo.

To test for an in vivo role of NaGST_(Nu), we generated an N. aromaticivorans mutant lacking Saro_2595 (12444Δ2595). Unlike its parent strain (12444Δ1879), 12444Δ2595 did not incorporate significant GGE into cell material in any culture (based on cell density and COD measurements; FIGS. 2A and B (panels C,I)) and Table 5). When provided only erythro-GGE, 12444Δ2595 produced only a small amount of MPHPV and threo-GGE in the medium (FIG. 2A (panel D)). When 12444Δ2595 was fed both vanillate and erythro-GGE, it completely metabolized the vanillate, and converted almost all of the GGE into MPHPV (FIG. 2B (panel J)). A small amount of guaiacol also appeared in the medium of this culture, suggesting that some MPHPV was cleaved by 12444Δ2595. However, unlike the situation for the parent strain, no extracellular HPV was detected in the 12444Δ2595 culture (FIGS. 2A and 2B (panels D,J)). These results show that NaGST_(Nu) is necessary for complete GGE metabolism by N. aromaticivorans.

NaGST_(Nu) is Sufficient for Conversion of GS-HPV into HPV in N. aromaticivorans.

To determine which step in the β-etherase pathway requires NaGST_(Nu), we incubated cell extracts of 12444Δ2595 and its parent strain (12444Δ1879) with racemic MPHPV and GSH. With the 12444Δ1879 extract, MPHPV was converted to roughly equimolar amounts of guaiacol and HPV, along with a small amount of GS-HPV (FIG. 7 (A)). In contrast, the 12444Δ2595 extract incompletely cleaved MPHPV, producing roughly equimolar amounts of guaiacol and GS-HPV along with a low level of HPV (˜2% of the level of GS-HPV formed; FIG. 7 (B)). Thus, it appeared that the 12444Δ2595 extract was defective in the conversion of GS-HPV into HPV. When recombinant NaGST_(Nu) was added to the 12444Δ2595 extract (FIG. 7 (B)), the GS-HPV disappeared, with a concomitant increase in HPV, showing that the defect in GS-HPV cleavage by 12444Δ2595 extract was caused by the lack of NaGST_(Nu).

NaGST_(Nu) homologues cleave β(R)- and β(S)-GS-HPV. The GST Nu-class is a widespread protein family (Stourman et al. 2011, Mashiyama et al. 2014). Indeed, a non-redundant BLAST search of the NCBI database using NaGST_(Nu) as query identified over 1,000 proteins with amino acid identities of >61% and E-values <2.0×10⁻¹²⁴. Besides GST3 from Novosphingobium sp. MBESO4 (Ohta et al. 2015), the only other Nu-class GSTs that have been analyzed for catalytic activity are E. coli ecYghU and ecYfcG (61% and 42% amino acid sequence identity with NaGST_(Nu), respectively; FIG. 5). The roles of these enzymes are unknown, but they are reported to have disulfide bond oxidoreductase activity in vitro (Stourman et al. 2011, Wadington et al. 2009, Wadington et al. 2010, Mashiyama et al. 2014). Though E. coli is not known to metabolize lignin-derived GSH adducts, we found that recombinant ecYghU and ecYfcG were both able to cleave β(R)- and β(S)-GS-HPV in vitro (FIG. 6). While the K_(M) values for ecYghU were comparable to those of NaGST_(Nu) (Table 7), its k_(cat) values were much lower than those of NaGST_(Nu), resulting in k_(cat)/K_(M) values for ecYghU with GS-HPV ˜100-fold lower than those of NaGST_(Nu) under our assay conditions (Table 7). For ecYfcG, K_(M) values were ˜10-fold higher and k_(cat) values were ˜10-fold lower than those of ecYghU, leading to k_(cat)/K_(M) values with GS-HPV ˜100-fold lower for ecYfcG than for ecYghU.

To test whether ecYghU can function in the β-etherase pathway in vivo, we created a strain of N. aromaticivorans in which Saro_2595 was replaced in the genome by the E. coli yghU gene. The resulting strain, 12444ecyghU, metabolized GGE slower than 12444Δ1879 (the strain containing NaGST_(Nu); FIGS. 2A and 2B), but still removed all of the GGE from the medium and assimilated it into biomass, whereas 12444Δ2595 could not (FIGS. 2A and 2B (panels E,F,K,L); Table 5). This shows that ecYghU can substitute for NaGST_(Nu) in the N. aromaticivorans β-etherase pathway.

Although the sphingomonad Sphingobium sp. SYK-6 can metabolize GGE (Palamuru et al. 2015, Sato et al. 2009), no enzyme capable of cleaving β(S)-GS-HPV has been identified in this organism (Masai et al. 2003). We tested a recombinant version of the Nu-class GST from Sphingobium sp. SYK-6, coded for by SLG_04120 and named here SYK6GST_(Nu), and found that it cleaved both β(S)- and β(R)-GS-HPV in vitro (FIG. 6). SYK6GST_(Nu) had higher k_(cat) and lower K_(M) with β(S)-GS-HPV than with β(R)-GS-HPV, leading to a ˜10-fold greater k_(cat)/K_(M) with the β(S)-isomer (Table 7). Thus, SYK6GST_(Nu) could cleave β(S)-GS-HPV and potentially contribute, along with LigG, to β(R)-GS-HPV cleavage in Sphingobium sp. SYK-6 (Masai et al. 2003).

Structural Characterization of NaGST_(Nu).

We solved two structures of NaGST_(Nu), crystallized under different conditions, with resolutions of 1.25 (pdb 5uuo) and 1.45 (pdb 5uun) Å (Table 8). The structures align with each other with an RMS distance of 0.108 over 7381 atoms. NaGST_(Nu) is a homo-dimer; each subunit contains a characteristic N-terminal GST (thioredoxin-like) domain (Val39 to Gly129), a C-terminal GST domain (Ser135 to Leu257), and a C-terminal extension not present in most other characterized classes of GSTs (Val258 to Phe288)* (FIG. 8 (A)).

TABLE 8 Statistics for the crystal structure determinations of NaGST_(Nu). PDB entry 5uuo 5uun Precipitant Ammonium sulfate Ammonium acetate Wavelength 0.7749 1.033 Resolution range 29.36-1.25 (1.295-1.25) 43.97-1.45 (1.502-1.45) Space group P 21 21 21 P 21 21 21 Unit cell 68.81 70.39 68.59 70.64 168.23 90 90 90 168.57 90 90 90 Total reflections 3049254 (309691) 1854065 (113574) Unique reflections 225346 (22262) 140421 (11067) Multiplicity 13.5 (13.9) 13.2 (10.3) Completeness (%) 99.91 (99.70) 96.48 (76.88) Mean I/sigma(I) 23.67 (1.96) 36.36 (8.82) Wilson B-factor 16.03 13.23 R-Merge 0.05235 (1.297) 0.04367 (0.2047) R-Means 0.05445 (1.346) 0.04544 (0.2159) R-Pim 0.01485 (0.3565) 0.01239 (0.06599) CC1/2 1 (0.841) 1 (0.986) CC* 1 (0.956) 1 (0.997) Reflections used 225262 (22242) 140408 (11066) in refinement Reflections used 1963 (197) 1852 (154) for R-free R-Work 0.1303 (0.2607) 0.1357 (0.206) R-Free 0.1324 (0.2979) 0.1365 (0.1595) CC(work) 0.978 (0.913) 0.978 (0.971) CC(free) 0.988 (0.851) 0.975 (0.9363) Non-H atoms 5658 5852 Macromolecules 4626 4628 Ligands 252 244 Solvent 780 980 Protein residues 569 1040 RMS(bonds) 0.007 0.008 RMS(angles) 1.02 1.08 Ramachandran 93.75/2.30/0.35 96.82/2.65/0.53 favored/allowed/ outliers (%) Rotamer 0.43 0.43 outliers (%) Clashscore 1.89 2.13 Average B-factor 22.39 19.61 B-Factor 20.02/26.98/34.97 16.91/19.017/33.64 macromolecules/ ligands/solvent No. TLS groups 9 9 *The residue numbers used here for NaGST_(Nu) are for the native protein represented by SEQ ID NO: 18; residues numbers in the pdb entries differ by +5 since the protein used for crystallization contained an N-terminal extension left after proteolytic removal of the His₈-tag. Statistics for the highest-resolution shell are shown in parentheses.

The structure of NaGST_(Nu) is most similar to those of the Nu-class GSTs ecYghU (pdb 3c8e (Stourman et al. 2011), with an RMSD of 0.49 Å over 3116 atoms) and Streptococcus sanguinis SK36 YghU (ssYghU; pdb 4mzw (Patskovsky et al.), with an RMSD of 0.58 Å over 3004 atoms). In NaGST_(Nu), ecYghU, and ssYghU, a short channel leads from the active site pocket to the solvent (FIGS. 9 and 12). In other structurally characterized Nu-class GSTs, including ecYfcG (pdb 3gx0 (Wadington et al. 2009)), the active site is more solvent exposed, since these proteins lack N-terminal residues that contribute to the channel walls in the other proteins (FIG. 5; Supplemental Materials of Thuillier et al. 2013).

The active site electron density in NaGST_(Nu) was modeled as a mixed population of GSH1 and GSH2 thiols with an S—S distance of 2.4 Å (˜60% occupancy) and GS-SG disulfide with an S—S distance of 2.0 Å (˜40% occupancy) (FIG. 8 (B)). Since the crystallization solutions initially contained only GSH, the disulfide likely formed via adventitious oxidation of the closely situated GSH thiols during the crystallization period (˜4 weeks), or during X-ray diffraction data collection. For comparison, ecYghU shows a dithiol configuration with HS—SH distance of 2.8 Å (Stourman et al. 2011), whereas ssYghU and ecYfcG each show a disulfide configuration with S—S distance of 2.1 Å (Wadington et al. 2009, Patskovsky et al.).

In NaGST_(Nu), seven residues make close contacts with GSH1 (Thr51, Asn53, Gln86, Lys99, Ile100, Glu116, and Ser117) and three residues make close contacts with GSH2 (Asn25, Asn53, and Arg177 from the opposite chain in the dimer) (FIG. 8 (B)). These residues and contacts to the GSHs are conserved in the YghU structures ((Stourman et al. 2011, Patskovsky et al.); ecYfcG is missing an analogue of Asn25, as it is truncated at its N-terminus relative to the other proteins (FIG. 5). The conserved threonine forms a hydrogen-bond with the GSH1 thiol that is characteristic of Nu-class GSTs (Stourman et al. 2011) (FIG. 8 (B); 3.0 Å interatomic distance for Thr 51 in NaGST_(Nu)). ecYghU is predicted to have one additional interaction with GSH1 (via Gln151 (Stourman et al. 2011)) not present in NaGST_(Nu), ssYghU, or ecYfcG (which contain Val150, Thr148, and Val105 at this position, respectively).

Alignment of the individual subunits of NaGST_(Nu), ecYghU and ssYghU shows four different configurations for their C-termini (FIG. 9 (A)): two for NaGST_(Nu) (FIG. 9 (B,C)) and one each for ecYghU (FIG. 9 (D)) and ssYghU (FIG. 9 (E)). The C-terminal region of ecYfcG is in essentially the same configuration as that of ssYghU, though the last eleven residues of ecYfcG are not present in its crystal structure. The two NaGST_(Nu) subunits differ in the positioning of residues Leu258 to Phe288, with Phe288 being distant from the active site channel in one subunit (5uun open, FIG. 9 (B)), and near the channel entrance in the other (5uun closed, FIG. 9 (C)), a difference of ˜18 Å. The closed NaGST_(Nu) configuration is stabilized by hydrogen-bonds between the sidechain amide of Lys262 and the carbonyl oxygen of Lys286, and the sidechain amide of Lys286 and the carbonyl oxygen of Arg220. The open configuration lacks these interactions and the sidechain of Arg220 has two rotamer positions. The sizes of the active site channel opening in the open and closed configurations of NaGST_(Nu) are ˜18 Å² and ˜11 Å², respectively (FIG. 9 (B,C)). For comparison, the area of active site access for both ecYghU and ssYghU is ˜25 Å² (FIG. 9 (D,E)). The placement of helices-10 and 11 in ecYghU (FIG. 9 (D)), and the truncated C-terminal sequence of ssYghU (FIG. 9 (D), FIG. 5), make it is unlikely that either of these C-termini can occupy a position similar to that seen in NaGST_(Nu).

Modeling of β(R)- and β(S)-GS-HPV into the GSH2 position in the NaGST_(Nu) active site predicts that their HPV moieties extend into the active site channel in different orientations (FIG. 10 (A)). The channel interior includes the phenol groups of Tyr166 and Tyr224, and the carboxylate of Phe288 in the closed configuration. Our models predict that these three residues interact differently with β(R)- and β(S)-GS-HPV. With the less reactive β(R)-GS-HPV, Tyr166 is predicted to provide a long interaction (4 Å) with the α-ketone, whereas Tyr224 hydrogen-bonds to the γ-hydroxyl (3.1 Å), which in turn hydrogen-bonds to the α-ketone (FIG. 10 (B)). With the more reactive β(S)-GS-HPV, Tyr166 is predicted to provide a cation-n interaction with its aromatic ring (average distance 3.4 Å), whereas Tyr224 provides a hydrogen-bond to its γ-hydroxyl (FIG. 10 (C)). Phe288 is also predicted to hydrogen-bond with the phenolic group of both β(R)- and β(S)-GS-HPV in the closed configuration of NaGST_(Nu).

Discussion

In developing bio-based systems to depolymerize lignin, optimized cellular and enzyme catalysts are needed. In this study, we tested sphingomonads for the ability to break the β-aryl ether bond commonly found in lignin (FIG. 1), and characterized a Nu-class glutathione-S-transferase from N. aromaticivorans that acts as a glutathione lyase in the β-etherase pathway (NaGST_(Nu)). Our finding that other Nu-class GSTs also catalyze this reaction provides important insight into the function of this enzyme family in lignin depolymerization and possibly metabolism of other compounds that proceed via GSH-conjugates.

Differences in GGE Metabolism.

We found that N. aromaticivorans was the most effective species studied here at metabolizing the dimeric aromatic compound GGE and assimilating it into cellular material. The rate of GGE metabolism by N. aromaticivorans is comparable to those of Erythrobacter sp. SG61-1L (Palamuru et al. 2015) and Novosphingobium sp. MBESO4 (Ohta et al. 2015) (FIG. 3). Sphingobium sp. SYK-6 (Palamuru et al. 2015), Novosphingobium sp. PP1Y (FIG. 4A), and S. xenophagum (FIG. 4B) are slower and/or less efficient at metabolizing GGE, even though they each contain enzymes implicated in the β-etherase pathway.

GGE Metabolism by N. aromaticivorans.

The appearance of extracellular MPHPV, threo-GGE, and HPV in N. aromaticivorans cultures suggests that it excretes these β-etherase pathway intermediates (FIGS. 2A and 2B). The appearance of threo-GGE in cultures fed erythro-GGE shows that GGE oxidation is reversible in vivo, as was previously found for Pseudomonas acidovorans D3 (Vicuña et al. 1987). The low level of extracellular guaiacol, and the absence of extracellular GS-HPV in GGE-fed cultures suggest that MPHPV cleavage occurred intracellularly, as was proposed for Novosphingobium sp. MBESO4 (Ohta et al. 2015), and which was expected, based on the requirement for GSH for this reaction (FIG. 1). Since the conversion of GS-HPV into HPV also requires GSH, this reaction also likely occurred intracellularly. The relatively late uptake of HPV from the media (FIGS. 2A and 2B (panels B,H)) suggests that N. aromaticivorans metabolized and assimilated guaiacol before HPV. In contrast, Erythrobacter sp. SG61-1L metabolized HPV, but not guaiacol (Palamura et al. 2015), and Pseudomonas acidovorans E-3 consumed the guaiacol only after consuming the phenylpropanoid formed from splitting veratrylglycerol-β-guaiacyl ether (Crawford et al. 1973). This shows that species use different strategies for metabolizing β-etherase pathway intermediates, a feature that could be useful in developing strains that produce specific pathway intermediates.

Bacterial metabolism of HPV has been proposed to proceed through acetovanillone, vanillin, vanillate, and protocatechuate (Crawford et al., 1973; Masai et al., 2007; Palamuru et al., 2015; Vicuña et al., 1987), and metabolism of guaiacol has been proposed to proceed through catechol (Crawford et al., 1973). However, we failed to detect any of these compounds in N. aromaticivorans culture media, suggesting that aromatic intermediates downstream of HPV and guaiacol were retained within the cells upon formation.

Nu-Class GSTs can Function as Glutathione Lyases.

We found that NaGST_(Nu), SYK6GST_(Nu), ecYghU, and ecYfcG cleave the GS-moiety from both β(R)- and β(S)-GS-HPV, though with a wide range of catalytic efficiencies (k_(cat)/K_(M)) (at least 10⁴-fold; Table 7). Thus, along with GST3 from Novosphingobium sp. MBESO4 (Ohta et al., 2015), all five of the Nu-class GSTs that have been tested for GS-HPV cleavage show glutathione lyase (deglutathionylation) activity with both stereoisomers of this substrate. Phylogenetic analysis of Nu-class GSTs shows these enzymes lie in widely separate sub-clades, suggesting that this activity may be widespread throughout this large class of proteins (FIG. 13).

Proposed Mechanism for the Nu-Class GST Glutathione Lyase Reaction.

We modeled the GS-moiety of GS-HPV into the GSH2 active site position of NaGST_(Nu) (FIG. 8 (B)), with the HPV moieties of β(R)- and β(S)-GS-HPV extending into the active site channel in different orientations (FIG. 10(A) and FIG. 12 (A,B)). We propose a mechanism for the glutathione lyase reaction (FIG. 11) in which the thiol of the GSH1 molecule (FIG. 8 (B)) is activated by hydrogen-bonding with the conserved active site threonine (Thr51), which has moderate reactivity as a base at pH 8.5 (FIG. 11 (A)). The activated GS1 thiol attacks the GS- of GS-HPV to form a disulfide GS-SG. In our proposed mechanism, the lyase reaction (C—S bond cleavage) is facilitated by polarization of the GS-HPV α-ketone by interactions involving Tyr166 and Tyr224 (which are highly conserved amongst many Nu-class GSTs; FIG. 5), and the γ-hydroxyl of HPV, with different interactions for the two GS-HPV stereoisomers (FIG. 11 (A)). With β(R)-GS-HPV, Tyr166 is predicted to provide a long interaction (3.9 Å) with the α-ketone, while Tyr224 hydrogen-bonds to the γ-hydroxyl (3.1 Å), which in turn hydrogen-bonds to the α-ketone (FIGS. 10 (B) and 11 (A)). Tyr166 is predicted to provide a cation-π interaction with the aromatic ring of the tighter binding β(S)-GS-HPV (average distance 3.4 Å), while Tyr224 provides a hydrogen-bond to its γ-hydroxyl (FIGS. 10 (C) and 11 (A)). Phe288 is also predicted to hydrogen-bond with the phenolic group of both β(R)- and β(S)-GS-HPV in the closed C-terminal configuration of NaGST_(Nu) (FIGS. 10 (B,C) and 11). ecYfcG lacks analogues of these Tyr residues (FIG. 5), which may contribute to its diminished catalytic capability (k_(cat)/K_(M) values ˜10⁴-fold lower than those of NaGST_(Nu); Table 7). In the absence of a redox cofactor, we propose that a transient enolate (FIG. 11 (B)) stores the 2e⁻ reducing equivalents released by disulfide bond formation as an incipient carbanion. Due to active site steric constraints, our model places the reactive portion (S—C_(β)—(C_(α)═O)-aryl) of both β(R)- and β(S)-GS-HPV into roughly planar configurations in the NaGST_(Nu) channel (FIG. 10 and FIG. 12 (A,B)), which should promote the formation of the enolate intermediate (FIG. 11 (B)). In contrast, steric constraints in the ecYghU active site channel resulting from differences between its channel interior and that of NaGST_(Nu) place these reactive GS-HPV atoms ˜45° out of alignment in our ecYghU models (FIG. 12 (C,D)), providing a possible explanation for the slower reactivity of ecYghU with GS-HPV compared to NaGST_(Nu) (ecYghU has values of k_(cat)/K_(M) ˜100-fold lower than those of NaGST_(Nu); Table 7). Collapse of the proposed enolate intermediate proceeds with carbanion trapping of a solvent-derived proton, corresponding to reduction of the carbon atom originally containing the thioether bond (FIG. 11 (C)).

This proposed mechanism for Nu-class GSTs with GS-HPV is different from that proposed for the omega class GST LigG, in which β(R)-GS-HPV initially forms a mixed disulfide with a cysteine residue, releasing the HPV moiety (Pereira et al. 2016). A GSH molecule then enters the LigG active site and combines with the enzyme bound GS-moiety to form GSSG. A side chain thiol is unlikely to be involved in Nu-class glutathione lyase activity, since NaGST_(Nu) only contains one cysteine, which is ˜21 Å away from the active site, and SYK6GST_(Nu) and ecYghU do not contain any cysteine residues.

NaGST_(Nu) Converts β(R)- and β(S)-GS-HPV into HPV in N. aromaticivorans.

Although GST3 from Novosphingobium sp. MBESO4 can convert β(R)- and β(S)-GS-HPV into HPV in vitro (Ohta et al. 2015), a physiological role in the β-etherase pathway has not been established. The inability of N. aromaticivorans 12444Δ2595 to completely metabolize GGE (FIGS. 2A and 2B (panels C,D,I,J)) shows that NaGST_(Nu) is necessary for the β-etherase pathway in N. aromaticivorans. Unexpectedly, we found that 12444Δ2595 accumulated extracellular MPHPV. Cleavage of MPHPV into guaiacol and GS-HPV is catalyzed by LigF and LigE/P (FIG. 1), enzymes that are likely expressed in 12444Δ2595, since crude extract from this strain can cleave MPHPV (FIG. 7 (B)). We hypothesize that without NaGST_(Nu), GS-HPV accumulates intracellularly in 12444Δ2595, and cells become limited for the GSH that is needed to cleave MPHPV.

It is unclear whether the trace amount of HPV formed in assays using 12444Δ2595 extract (FIG. 7 (B)) resulted from activity of an unknown enzyme or spontaneous cleavage of GS-HPV. In any event, addition of recombinant NaGST_(Nu) to the 12444Δ2595 extract resulted in the complete conversion of GS-HPV into HPV (FIG. 7 (B)), providing the first demonstration of a single enzyme being sufficient for one of the steps of the β-etherase pathway in vivo.

The Role of Nu-Class GSTs in the β-Etherase Pathway.

The ability of Nu-class GSTs to cleave both β(R)- and β(S)-GS-HPV raises the question of why some species contain both this enzyme and LigG (e.g. Sphingobium sp. SYK-6), which is specific for the β(R)-isomer (FIG. 1). Our data show that both NaGST_(Nu) and SYK6GST_(Nu) have a higher k_(cat)/K_(M) value with β(S)-GS-HPV than with the β(R)-isomer (Table 7). For NaGST_(Nu), this difference in k_(cat)/K_(M) values between the stereoisomers was ˜4-fold and the values (˜8000 and ˜1900 mM⁻¹ s⁻¹ for the β(S)- and β(R)-isomers, respectively) are both greater than that reported for LigG with β(R)-GS-HPV (1700 mM⁻¹ s⁻¹ (Pereira et al. 2016)). For SYK6GST_(Nu), the difference in k_(cat)/K_(M) values between the isomers is ˜10-fold, and the value with β(R)-GS-HPV (˜240 mM⁻¹ s⁻¹) is lower than that reported for LigG. These observations suggest that SYK6GST_(Nu) likely cleaves β(S)-GS-HPV in Sphingobium sp. SYK-6, but that LigG may play a role in cleaving β(R)-GS-HPV in that organism. Although cell extract from a Sphingobium sp. SYK-6 ΔLigG mutant completely cleaved a racemic GS-HPV sample (since the lysate presumably contained active SYK6GST_(Nu)) the physiological effect of deleting LigG was not reported (Masai et al. 2003). Perhaps organisms like N. aromaticivorans that lack LigG need Nu-class GSTs to have higher efficiencies toward β(R)-GS-HPV than Nu-class GSTs in species that contain a stereospecific enzyme like LigG.

The Potential Role(s) of Nu-Class GSTs in Bacteria that do not Contain the 13-Etherase Pathway.

Many organisms contain Nu-class GSTs, including those not known or predicted to use the β-etherase pathway (Mashiyama et al., 2014; Stourman et al., 2011). Whereas several of these enzymes have been found to have disulfide bond reductase activity in vitro, the physiological roles of most of these proteins are unknown (Mashiyama et al., 2014; Stourman et al., 2011). We found that ecYghU and ecYfcG from E. coli, an organism not known to metabolize lignin-derived phenylpropanoids, can cleave both β(R)- and β(S)-GS-HPV in vitro, though with lower catalytic efficiencies than NaGST_(Nu) and SYK6GST_(Nu) (Table 2). The fact that ecYghU can replace NaGST_(Nu) in N. aromaticivorans (FIGS. 2A and 2B (panels E,F,K,L)) shows that it can indeed function as a glutathione lyase in vivo. The relatively low k_(cat)/K_(M) values of ecYghU and ecYfcG in cleavage of GS-HPV compared to NaGST_(Nu) and SYK6GST_(Nu) could reflect the fact that GS-HPV is not a natural substrate for the E. coli enzymes. Although the overall structures of Nu-class GSTs are similar, differences in the residues surrounding the active sites (as seen between NaGST_(Nu) and ecYghU, for example; FIG. 12) could make enzymes from other organisms better suited for binding and cleaving other GS-conjugates that they more commonly encounter.

Conclusion

This work shows that N. aromaticivorans can rapidly and completely metabolize the β-aryl ether-containing compound GGE, and that the Nu-class glutathione-S-transferase NaGST_(Nu) plays a direct role in this process. The following example illustrates that NaGST_(Nu) can participate in cleavage of bona fide lignin oligomers in vitro, indicating utility of this enzyme in converting biomass into valuable chemicals. NaGST_(Nu) and other Nu-class GSTs can cleave both the β(R)- and β(S)-stereoisomers of the β-etherase pathway intermediate GS-HPV, in contrast to the other characterized enzymes in the pathway, which are stereospecific (FIG. 1). Our finding that ecYghU also cleaves GS-HPV shows that Nu-class GSTs from organisms lacking the β-etherase pathway can nevertheless act as racemic glutathione lyases.

Example 2

In Vitro Enzymatic Release of Syringyl, Guaiacyl, and Tricin Units from Lignin

Summary

New information and processes are needed to derive valuable compounds from renewable resources. Lignin is an abundant, heterogeneous, and racemic polymer in terrestrial plants, and it is comprised predominantly of guaiacyl and syringyl monoaromatic phenylpropanoid units that are covalently linked together in a purely chemical radical coupling polymerization process. In addition, the plant secondary metabolite, tricin, is a recently found and abundant lignin monomer in grasses. The most prevalent type of inter-unit linkage between guaiacyl, syringyl, and tricin units is the β-ether linkage. Previous studies have shown that enzymes in the bacterial β-etherase pathway catalyze glutathione-dependent cleavage of β-ether bonds in dimeric β-ether lignin model compounds, resulting in the release of monoaromatic products, the reduction of nicotinamide adenine dinucleotide (NAD⁺) to NADH, and the oxidation of glutathione (GSH) to glutathione disulfide (GSSG). To date, however, it remains unclear whether the known β-etherase enzymes are active on lignin polymers. Here, we report on enzymes that catalyze β-ether cleavage from model compounds and bona fide lignin, under conditions that recycle the cosubstrates NAD⁺ and GSH. Guaiacyl, syringyl and tricin derivatives were identified as reaction products when different model compounds or lignin fractions were used as substrates. These results provide the first demonstration of an in vitro enzymatic system that can recycle NAD⁺ and GSH while releasing aromatic monomers from model compounds as well as natural and engineered lignin oligomers. These findings can improve the ability to produce valuable aromatic compounds from a renewable resource like lignin.

Introduction

There is economic and environmental interest in using renewable resources as raw materials for production of chemicals that are currently derived from fossil fuels. Lignin, a renewable resource that accounts for ˜15-30% (dry weight) of vascular plant cell walls (Higuchi 1980, Lewis et al. 1990), is comprised of aromatic compounds that may be valuable commodities for the biofuel, chemical, cosmetic, food, and pharmaceutical industries (Sinha et al. 2008). Consequently, intensive efforts are currently aimed at developing chemical, enzymatic and hybrid methods for deriving simpler and lower molecular weight products from lignin (Gall et al. 2017).

The lignin backbone is predominantly composed of guaiacyl (G) and syringyl (S) phenylpropanoid units (FIG. 16 (A)) that derive from the monomers coniferyl and sinapyl alcohol that become covalently linked during lignification via radical coupling reactions, primarily by endwise addition of a monomer (radical) to the phenolic end of the growing polymer (radical). G and S units are inter-linked by a variety of chemical bonds by which the units are characterized: resinols (β-β), 4-O-5-diaryl ethers, phenylcoumarans (β-5), and β-O-4-aryl ethers (termed β-ethers hereafter)] (Adler 1977, Adler 1957, Adler 1955). In grasses, the flavone tricin (T units, FIG. 16 (A)) begins a chain and is covalently linked to the next unit via a 4-O-β-ether bond (Lan et al. 2016, Lan et al. 2015, del Rio et al. 2012). Given that approximately 50-70% of all inter-unit linkages in lignin are β-ethers (Adler 1977, Adler 1957, Adler 1955), cleavage of these bonds is crucial for processes aiming to derive valuable low molecular weight compounds from lignin in high yields. The formation of β-ether linkages during lignification generates a racemic lignin product containing both β(R)- and β(S)-carbons that, after re-aromatization of the quinone methide intermediate by proton-assisted water addition, are adjacent to either α(R)- or α(S)-configured benzylic alcohols (Akiyama et al. 2002, Sugimoto et al. 2002, Ralph et al. 1999). Each unit therefore has 4 optical isomers and two ‘real’ isomers—the so-designated threo and erythro (or syn and anti) isomers. Lignin depolymerization via β-ether bond cleavage has been demonstrated with chemical catalysis (Rahimi et al. 2013, Rahimi et al. 2014). In addition, cytoplasmic enzymes in a sphingomonad β-etherase pathway have been identified that oxidize and cleave model β-ether linked aromatic dimers (Masai et al. 2007).

The β-etherase pathway is present in Sphingobium sp. strain SYK-6 and other sphingomonads (e.g., Novosphingobium spp.) (Masai et al. 2007, Gall and Ralph et al. 2014). The diaromatic β-ether-linked guaiacylglycerol-β-guaiacyl ether (GGE, FIG. 16 (B)) lignin model compound has been used as a substrate to identify the following three enzymatic steps in cleavage of β-ether linkages in vitro (Gall and Ralph et al. 2014, Masai et al. 2003, Sato et al. 2009, Tanamura et al. 2010, Gall and Kim et al. 2014): (1) a set of dehydrogenases catalyze nicotinamide adenine dinucleotide (NAD⁺)-dependent α-oxidation of GGE to GGE-ketone and NADH (Sato et al. 2009, Masai and Kubota et al. 1993), (2) β-etherases that are members of the glutathione S-transferase superfamily, carry out glutathione (GSH)-dependent cleavage of GGE-ketone, releasing guaiacol and β-S-glutathionyl-γ-hydroxy-propiovanillone (GS-HPV) (Masai et al. 2003, Gall and Kim et al. 2014, Masai and Katayama et al. 1993), and (3) one or more glutathione lyases catalyze GSH-dependent cleavage of GS-HPV, yielding glutathione disulfide (GSSG) and γ-hydroxypropiovanillone (HPV) (Masai et al. 2003, Gall and Kim et al. 2014, Rosini et al. 2016, Kontur et al. 2018).

The use for multiple enzymes at some of the pathway's steps is attributable to the existence of both R- and S-configured chiral centers in lignin (Akiyama et al. 2002, Sugimoto et al. 2002, Ralph et al. 1999). The known NAD⁺-dependent dehydrogenases (LigD, LigL, LigN, and LigO) exhibit strict stereospecificity at the a position with indifference to the configuration at the β position (Sato et al. 2009). With model diaromatic substrates, LigD and LigO are active on the R-configured α-epimers, whereas LigL and LigN are active on the S-configured α-epimers. Because the combined activity of these dehydrogenases eliminates the chiral center at a, GGE-ketone exists as two β-enantiomers that are cleaved by stereospecific β-etherases LigE, LigP and LigF, each of which catalyzes the release of guaiacol with chiral inversion at the β position, and one of two β-epimers of GS-HPV (LigE and LigP convert β(R)-GGE-ketone to β(S)-GS-HPV and LigF converts β(S)-GGE-ketone to β(R)-GS-HPV) (Gall and Kim et al. 2014). The final step is the GSH-dependent cleavage of the GS-HPV epimers, yielding GSSG and HPV as coproducts. LigG has been shown to cleave both β(R)-GS-HPV and β(S)-GS-HPV (Rosini et al. 2016), although it appears to have a strong preference for the former (Masai et al. 2003, Gall and Kim et al. 2014). Recently, a GSH transferase from Novosphingobium aromaticivorans DSM12444 (NaGST_(NU); Saro_2595 in GenBank assembly GCA_000013325.1) (Kontur et al. 2018) has been shown to have high activity with β(R)-GS-HPV and β(S)-GS-HPV both in vivo and in vitro, producing HPV and GSSG as products (FIG. 16 (B)).

Despite what is known about the activity of individual β-etherase pathway enzymes with model diaromatic compounds, there is little information on their function with lignin oligomers. In vivo activity may be limited to aromatic dimers or small lignin oligomers due to restrictions in transporting large polymers into the bacterial cytoplasm where the β-etherase pathway enzymes are found. To better understand the function of β-etherase pathway enzymes, we sought to use a minimal set of enzymes to develop a coupled in vitro assay capable of releasing G, S and T aromatic monomers and recycling the cosubstrates NAD⁺ and GSH. Here we demonstrate complete conversion of GGE to guaiacol and HPV in a reaction containing LigD, LigN, LigE, LigF, NaGST_(Nu), and the Allochromatium vinosum DSM180 GSH reductase (AvGR), which catalyzes NADH-dependent reduction of GSSG (FIG. 16 (B)) (Reiter et al. 2013). We also show that this same combination of enzymes releases tricin from the model compound guaiacylglycerol-β-tricin ether (GTE, FIG. 16 (C)). In addition, we show that the same combination of enzymes releases G, S, and T units from bona fide lignin oligomers; this is the first report to demonstrate the release of tricin from lignin units by biological methods. We discuss new insights gained from this study and its implications for the future production of these and possibly other valuable products from lignin.

Methods

General.

GGE was purchased from TCI America (Portland, Oreg.). Tricin, GTE, GTE-ketone, HPV, γ-hydroxypropiosyringone (HPS) and GGE-ketone were synthesized by previously described methods (Adler et al. 1955, Lan et al. 2015, Masai et al. 1989). All other chemicals were purchased from Sigma-Aldrich (St. Louis, Mo.). Methods to isolate and characterize maize (Zea mays) corn stover (MCS) and hybrid poplar (HP) lignin samples were described previously (Lan et al. 2015, Stewart et al. 2009, Shuai et al. 2016). ¹H and ¹³C NMR spectra were recorded on a Bruker Biospin (Billerica, Mass.) AVANCE 700 MHz spectrometer fitted with a cryogenically-cooled 5-mm quadruple-resonance ¹H/³¹P/¹³C/¹⁵N QCI gradient probe with inverse geometry (proton coils closest to the sample). Manipulation of DNA and preparation of Escherichia coli transformant cultures were carried out according to previously described methods (Moore 2003). All lig genes from Sphingobium sp. strain SYK-6, as well as those encoding AvGR from A. vinosum DSM180 were codon optimized for expression in E. coli and obtained from GeneArt® (Life Technologies). NaGST_(Nu) was amplified and cloned from N. aromaticivorans DSM12444 genomic DNA (Kontur et al. 2018).

Plasmid and Protein Preparation.

Procedures for cloning, recombinant expression and purification of Tev protease, LigE, LigF, LigG, and NaGST_(Nu) are described elsewhere (Gall and Kim et al. 2014, Kontur et al. 2017). Codon-optimized ligD, ligN and genes AvGR were cloned into plasmid pVP302K (Gall and Kim et al. 2014) via the PCR overlap method (Shevchuk et al. 2004, Bryksin et al. 2010, Horton et al. 2013, Horton 1993). Expression and purification of LigD, LigN, NaGST_(Nu), and AvGR followed similar procedures as those used previously (Gall and Kim et al. 2014). Briefly, E. coli strain B834 cultures, transformed with expression plasmids, were grown aerobically overnight in 1 L of auto-induction ZYM-5052 medium (Studier 2005) supplemented with 100 μg mL⁻¹ kanamycin. Cells were pelleted and extracts prepared via compression and sonication. Histidine-tagged proteins were purified from cell lysates via Ni-NTA affinity chromatography with QIAGEN Ni-NTA resin. His-tagged Tev protease was used to liberate N-terminal His-tags and a second round of Ni-NTA affinity chromatography was used to remove the tag and Tev protease before separation by size-exclusion chromatography. Protein preparations were concentrated and frozen with liquid N₂.

Enzyme assays. In vitro enzyme assays containing LigD, LigN, LigE, LigF, NaGST_(Nu) (or LigG), and AvGR (or a subset of those enzymes) were conducted in assay buffer (25 mM Tris, 2.0% DMSO, pH 8.0). The concentration of each enzyme was 50 μg mL⁻¹ in all assays. When GGE (6 mM) was the substrate, the initial cosubstrate concentrations were 2 mM NAD⁺ and 4 mM GSH. When GTE (1 mM) was the substrate, the initial cosubstrate concentrations were 5 mM NAD⁺ and 5 mM GSH. When an isolated lignin sample was used as the substrate (2.2 mg mL⁻¹), the initial cosubstrate concentrations were 2 mM NAD⁺ and 4 mM GSH. Enzyme assays (1 mL or larger volume as needed) were carried out (in duplicate) as follows: (1) the substrate (GGE, GTE, or lignin) was dissolved in DMSO (50-times concentrated above the intended assay concentration) and 20 μL of the solution were added to a 2 mL vial, (2) 880 μL of 25.6 mM Tris pH 11.5 (where the acidic effect of GSH drops the pH to 8.0 after addition of 5 mM GSH), (3) 50 μL of a stock solution in 25 mM Tris containing NAD⁺ and GSH (each is 20-times concentrated above the intended assay concentration), and (4) 50 μL of 20-times concentrated mixture of the desired enzymes. At indicated time points, 150 μL samples were removed from an assay and enzymatic activity was abolished by pipetting into 5 μL of 5 M phosphoric acid. GGE, guaiacol, HPV, and HPS concentrations were quantified for each time point [see below] using a linear regression of known standards for each compound.

Preparative Gel-Permeation Chromatography (GPC).

GPC of lignin samples was carried out using a Beckman 125NM solvent delivery module equipped with a Beckman 168 UV detector (λ=280 nm) and a 30 mL Bio-Rad Bio Bead S-X3 column (a neutral, porous styrene-divinylbenzene copolymer). Dimethylformamide (DMF) was used as the mobile phase at a flow rate of 1.0 mL min⁻¹. Between 20 and 50 mg of lignin was dissolved in a minimal amount of DMF, injected into the mobile phase, and 1 mL fractions were collected until UV absorption decreased to baseline levels. Fractions were then subjected to analytical GPC to estimate their average molecular weight (MW). The DMF was evaporated in vacuo in order to recover material used for enzyme assays.

Analytical GPC.

Analytical GPC of lignin samples was carried out with a Shimadzu Prominence Ultra Fast Liquid Chromatography system (LC-20AD pumps, SIL-20AC HT autosampler, CTO-20A column oven and CBM-20A controller) and using two TSKgel Alpha-2500 (300×7.8 mm; Tosoh Bioscience) columns at 40° C. Samples (10 μL injection volume) containing approximately 1 mg mL⁻¹ of isolated or GPC-fractionated lignin were injected into a mobile phase (100 μM LiBr in DMF) at a flow rate of 0.3 mL min⁻¹ with a run length of 90 min. An SPD-M20A photodiode array detector (λ=200 nm) was used for the determination of elution times that were subsequently converted to MW values using regression analysis of ReadyCal-Kit Polystyrene standards.

C₁₈-Chromatography.

C₁₈-Chromatographic separations were carried out using a Beckman 125NM solvent delivery module equipped with a Beckman 168 UV detector. 150 μL samples from enzyme assays were collected and 20 μL aliquots were injected into either a 4×120 mm Restek Ultra Aqueous C₁₈-reversed stationary phase column, or a 4.6×250 mm Phenomenex Luna 5u C₁₈(2)-reversed stationary phase column with a 1.0 mL min⁻¹ mobile phase composed of a mixture of an aqueous buffer (5 mM formic acid in 95/5 H₂O/acetonitrile) and methanol. Samples from enzyme assays using GTE as the substrate were analyzed on the Phenomenex column to improve separation of GTE and tricin. All other C₁₈-chromatographic separations were carried out using the Restek column. For the Restek column, the methanol fraction of the buffer (with water as the remainder) was adjusted as follows: 0-6 min, 30% methanol; 6-15 min, gradient from 30 to 80% methanol; 15-27 min, 80% methanol; 27-28 min, gradient from 80 to 30% methanol; 28-33 min, 30% methanol. For the Phenomenex column, the gradient system was as follows: 0-6 min, 10% methanol; 6-50 min, gradient from 10 to 90% methanol; 50-63 min, 90% methanol; 63-64 min, gradient from 90 to 10% methanol; 64-70 min, 10% methanol.

Results Design of a Coupled In Vitro Assay for Cleavage of β-Ether-Linked Diaromatic Compounds.

As an initial substrate for this assay we used erythro-GGE, which is a mixture of enantiomers (αR,βS)-GGE and (αS,βR)-GGE that has been used extensively as a substrate with β-etherase pathway enzymes vitro (Gall and Ralph et al. 2014, Masai et al. 2003, Sato et al. 2009, Tanamura et al. 2010, Gall and Kim et al. 2014). We used recombinant preparations of LigD and LigN as these dehydrogenases are reported to be sufficient for the NAD⁺-dependent oxidation of R- and S-configured α-anomers of erythro-GGE in vitro (Sato et al. 2009). The assay also contained recombinant preparations of LigE and LigF that have been shown to separately catalyze the GSH-dependent conversion of a racemic mixture of GGE to guaiacol and the S- and R-epimers of GSH-HPV (Gall and Kim et al. 2014). NaGST_(Nu) was present to catalyze the GSH-dependent cleavage of the GSH-HPV epimers to HPV and GSSG. The properties of individual enzymes (FIG. 16 (B)) predicts that this coupled system will require equimolar concentrations of GGE and NAD⁺ and twice as much GSH for complete conversion of GGE to HPV and guaiacol.

In an attempt to reduce the amount of added NAD⁺ and GSH that would be needed for full conversion of diaromatic substrate to products, some reactions included recombinant AvGR, which catalyzes the NADH-dependent reduction of GSSG (Reiter et al. 2013), thereby recycling the cosubstrates NAD⁺ and GSH for continued conversion of the β-ether substrates. This cosubstrate recycling system was tested with 6 mM erythro-GGE and limiting concentrations of NAD⁺ (2 mM) and GSH (4 mM) (FIG. 17A (panel A)). Using a mixture of LigD, LigN, LigE, LigF, and NaGST_(Nu) (without AvGR), we observed that erythro-GGE was partially converted to HPV and guaiacol (FIG. 17A (panel B)). Quantification of this assay revealed that the erythro-GGE concentration decreased from 6.0 mM to 3.8 mM at the end of the assay, whereas the HPV and guaiacol concentrations were each 2.0 mM, the NAD⁺ levels were non-detectable, and the threo-GGE [a mixture of enantiomers (αR,βR)-GGE and (αS,βS)-GGE] concentration increased to 0.1 mM presumably due to the reported reversibility of the LigD/LigN reactions (Pereira et al. 2016). Thus, the final GGE concentration (3.9 mM, the sum of erythro-GGE and threo-GGE concentrations) was consistent with consumption of 2.0 mM NAD⁺. In addition, the production of 2.0 mM (each) of HPV and guaiacol was consistent with the consumption of 4.0 mM GSH, where 2.0 mM GSH was consumed in the LigE/LigF reactions and an additional 2.0 mM GSH was consumed in the NaGST_(Nu) reaction.

To test the impact of AvGR on this assay, we added it to a parallel in vitro reaction. In the presence of AvGR (FIG. 17A (panel C)), we found that GGE was completely consumed along with the appearance of equimolar amounts of HPV and guaiacol (6.0 mM each) without a detectable change in the NAD⁺ concentration or accumulation of any β-etherase pathway intermediates by the time of the assay's conclusion. To determine if any β-etherase pathway intermediates accumulated over the course of the assay, we tested for time-dependent changes in the concentrations of the substrate, known pathway intermediates and products in a parallel reaction (FIG. 18). We found that as erythro-GGE degradation occurs there is a time-dependent accumulation and depletion of GGE-ketone and threo-GGE and, eventually, complete equimolar conversion of the substrate to HPV and guaiacol (FIG. 17A (panels A-C)). From these results, we conclude that the combination of LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR is sufficient to process all of the chiral centers in a β-ether substrate such as erythro-GGE. In addition, we conclude that the presence of AvGR is sufficient to recycle the cosubstrates NAD⁺ and GSH that are needed for cleavage of β-ether bonds in a model diaromatic compound such as erythro-GGE.

From information available in the literature, it has remained unclear whether the GSH lyase from Sphingobium strain SYK-6, LigG, exhibits a preference for β(R)-GS-HPV (Masai et al. 2003, Gall and Kim et al. 2014), or is capable of cleaving the thioether linkages in both β(R)-GS-HPV and β(S)-GS-HPV (Rosini et al. 2016). As the presence of NaGST_(Nu) resulted in cleavage of both β(R)-GS-HPV and β(S)-GS-HPV in this coupled reaction system (FIG. 17A (panels A-C)), we sought to use this in vitro assay to test the activity of LigG under identical conditions. When we performed an assay using 6.0 mM erythro-GGE, 2.0 mM NAD⁺, and 4.0 mM GSH, as well as the mixture of LigD, LigN, LigE, LigF, and LigG (without AvGR), we observed partial conversion of GGE to HPV and guaiacol (FIG. 17B (panel D)). At the end of this assay, the total GGE concentration (4.0 mM, the sum of erythro-GGE and threo-GGE concentrations) was expected based on the consumption of 2.0 mM NAD⁺. Further, the production of 2.0 mM (each) of HPV and guaiacol was consistent with the consumption of 4.0 mM GSH (2.0 mM GSH consumed by each of the LigE/LigF and LigG reaction steps). When we added AvGR to a parallel reaction that contained GGE (6.0 mM), NAD⁺ (2 mM), GSH (4 mM) and a combination of LigD, LigN, LigE, LigF, and LigG, we did not observe complete conversion of GGE to HPV and guaiacol (FIG. 17B (panel E)). Instead, we detected the diaromatic substrate (erythro-GGE), threo-GGE, GS-HPV, and GGE-ketone (0.7 mM). In contrast to what is found when NaGST_(Nu) was present under identical reaction conditions, the presence of LigG led to incomplete utilization of the diaromatic substrate and the accumulation of β-etherase pathway intermediates. From these results we conclude that LigG is not able to completely cleave both β-epimers of GS-HPV in vitro. Consequently, all subsequent assays were performed using NaGST_(Nu) as a source of GSH lyase activity.

Production of Tricin from GTE In Vitro.

In grasses, the flavone tricin (T, FIG. 16 (A)) is covalently linked to one end of lignin, via a β-ether bond (Lan et al. 2016, Lan et al. 2015, Lan et al. 2014). Although β-etherase pathway enzymes have been shown to cleave β-ether-linked diaromatic model compounds containing G and S monomers, to date there is no published data on their ability to remove the diaromatic flavonoid T units from any substrate. Thus, we sought to test the ability of the coupled assay to cleave GTE (FIG. 16 (C)), a model compound containing a β-ether linked tricin moiety. HPLC analysis of the synthetic GTE (FIG. 19A (A)) indicated that it contained a 6:1 ratio of erythro-GTE [(αR,βS)-GTE and (αS,βR)-GTE] to threo-GTE [(αR,βR)-GTE and (αS,βS)-GTE], which was consistent with the NMR analysis of this material (Lan et al. 2016).

When we incubated 1.0 mM GTE, 5.0 mM NAD⁺, 5.0 mM GSH with the combination of LigD, LigN, LigE, LigF and NaGST_(Nu) (FIG. 19B (B)) we observed the complete conversion of GTE to tricin and HPV. This result predicts that LigD and LigN oxidize GTE to form GTE-ketone, LigE and LigF catalyze β-ether cleavage in GTE-ketone to form GS-HPV and tricin, and NaGST_(Nu) releases HPV from GS-HPV (FIG. 16 (C)), suggesting that the larger β-ether-linked flavone model was able to access the active sites in LigD, LigN, LigE, and LigF. To further test this hypothesis, we assayed for the presence of the expected β-etherase pathway intermediates, GS-HPV and GTE-ketone, from GTE. By performing a parallel reaction containing the same substrates and only LigD, LigN, LigE, and LigF (FIG. 19B (C)), we observed that GTE was degraded and tricin was produced. However, in this assay, there was no detectable production of HPV and we observed accumulation of GS-HPV. These findings indicate that the absence of NaGST_(Nu) prevented the conversion of GS-HPV to HPV (FIG. 16 (C)). Finally, in an assay containing only the enzymes LigD and LigN (FIG. 19B (D)), we found that GTE was almost completely converted to GTE-ketone, as expected from the NAD⁺-dependent α-oxidation activity of GTE. Together, the data show for the first time that T units can be derived from β-ether-linked model compounds in vitro using enzymes, cosubstrates and intermediates that are known to be part of the β-etherase pathway (FIG. 16).

Release of G, S, and T Units from Lignin Oligomers.

With the coupled enzymatic system in place, we tested it for activity with lignin oligomers. First, we tested if a mixture of LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR produced S units from a high-syringyl hybrid poplar (HP) lignin polymer (Stewart et al. 2009, Shuai et al. 2016). To ensure that the test was performed with lignin oligomers rather than low-MW material, we fractionated the HP lignin by GPC and pooled the high-MW fractions (FIG. 20, Table 9) for use as a substrate (FIG. 21 (A-B)). From 2.2 mg mL⁻¹ lignin oligomers having MW between 9,000 and 12,000 (with 2 mM NAD⁺ and 4 mM GSH), we detected the production of 1.0 mM HPS, the HPV analog expected to be produced by cleavage of β-ether bonds from a syringyl unit at one end of the lignin chain. We also detected the formation of an unknown product in this reaction (FIG. 21 (B)), which could be either a chemically modified S unit released from the HP lignin or a GS-linked intermediate product. Furthermore, syringaresinol, a dimeric unit in the HP lignin polymer (Stewart et al. 2009), was not detected as a product of the enzymatic reaction.

TABLE 9 Estimated size of the HP lignin fractions after preparative GPC (size distributions are shown in FIG. 9-2). Size was determined from analytical GPC and is reported in Da and the corresponding polymer length is reported in number of units, based on the MW of syringaresinol (418.44) and β-ether-linked syringyl units (228.24). Average Average MW Length (Units) Original sample, pre-fractionation 8,665 38.3 Fraction 1* 11,550 51.0 Fraction 2* 10,780 47.6 Fraction 3* 9,340 41.3 Fraction 4 7,240 32.0 Fraction 5 5,200 23.0 Fraction 6 3,720 16.5 Fraction 7 2,660 11.8 Fraction 8 1,910 8.5 Fraction 9 1,280 5.8 Asterisks highlight fractions that were pooled and used as the substrate in enzyme assays.

Given the ability of the enzymatic assay to release HPS from HP lignin, we also tested for the release of aromatic monomers from a more complex lignin, such as the one derived from maize corn stover (MCS) (Lan et al. 2016, Lan et al. 2015). To generate substrates for these assays, we used preparative GPC to size-fractionate MCS lignin (FIG. 22, Table 10) and tested materials with different apparent MW values as the source of lignin oligomer substrates for enzyme assays (FIGS. 23A and 23B). To test for activity with these samples, we incubated LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR with MCS lignin oligomers (2.2 mg mL⁻¹), 2.0 mM NAD⁺ and 4.0 mM GSH (FIG. 23A (panel A)). In these experiments, we detected release of HPV and HPS in assays using lignin oligomers with average MW ranging from 460 to 10,710 (FIG. 22). The highest concentrations of HPV (0.4 mM) and HPS (0.1 mM) were observed with lignin oligomers having an average MW of 1,390 (FIG. 23B (panel D)) as the substrate. In general, larger lignin oligomers resulted in lower accumulation of HPV and HPS. In addition, similar to the observations with HP lignin, unknown products were detected in most of the enzymatic reactions with the different lignin fractions (FIGS. 23A and 23B). Tricin was only observed as a reaction product when using the lowest MW fraction tested (MW=460, FIG. 23B (panel F)). In sum, we conclude from these experiments that a combination of LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR can release some, but not all, G, S and T units from MCS lignin oligomers.

TABLE 10 Estimate size of the MCS lignin fractions after preparative GPC (size distributions are shown in FIG. 5). Size was determined from analytical GPC and is reported in Da and the corresponding polymer length is reported in number of units, based on the crude assumption that the average unit has a MW of 210. Average Average MW Length (Units) Original sample, pre-fractionation 5,980 28.5 Fraction 1* 10,710 51.0 Fraction 2 9,860 46.9 Fraction 3 8,320 39.6 Fraction 4 6,690 31.9 Fraction 5* 5,370 25.6 Fraction 6 3,930 18.7 Fraction 7 2,110 10.1 Fraction 8* 1,390 6.6 Fraction 11 880 4.2 Fraction 14* 660 3.1 Fraction 17* 460 2.2 Asterisks highlight fractions that were used as the substrate in enzyme assays.

Discussion

In order to use a polymer like lignin as a source of valuable aromatics and other chemicals it is necessary to develop new or improve on existing depolymerization strategies. There has been considerable interest in exploring the use of the bacterial β-etherase pathway for the biological production of aromatics from this renewable plant polymer. There is now a large amount of information on the types of model diaromatic substrates recognized by individual β-etherase enzymes in vitro, the products of their activity, and their structural or functional relationships to other known enzymes (Pereira et al. 2016, Helmich et al. 2016). Despite this, information is lacking on their activity with lignin oligomers. In addition, as these are cytoplasmic enzymes, it is plausible that they evolved to break down β-ether links only in the smaller lignin oligomers that could be transported inside the cells. In this work, we sought to develop a coupled in vitro system containing a set of β-etherase pathway enzymes that was capable of releasing monoaromatic compounds when incubated with different substrates. We reasoned that such a system would provide additional information on the β-etherase enzymes and aid in studies aimed at determining the requirements for release of valuable aromatics from bona fide lignin oligomers.

In this study, we identified a minimum set of enzymes (LigD, LigN, LigE, LigF, NaGST_(Nu), and AvGR) that is capable of cleaving β-ether linkages and completely converting model diaromatic compounds to aromatic monomers. We further showed that this coupled in vitro assay system is capable of stoichiometric production of monoaromatic products from model diaromatics in the presence of limiting amounts of the cosubstrates NAD⁺ and GSH. The ability to recycle NAD⁺ and GSH reduces the need for expensive cofactors and increases the future utility of a coupled enzyme system for processing lignin oligomers in vitro. Finally, we showed that this coupled enzyme system has activity with fractionated lignin oligomers. Below we summarize the new information gained from using this assay with widely used or new model β-ether linked substrates as well as lignin oligomers of different sizes.

Insights Gained from Using the Coupled Assay with Diaromatic Compounds.

Using GGE as a substrate, we demonstrated that the GSH reductase AvGR is capable of recycling the cosubstrates NAD⁺ and GSH, enabling the β-etherase enzymes to completely cleave GGE in the presence of sub-stoichiometric amounts of these cofactors (FIG. 17A (panels A-C)). The A. vinosum DSM180 AvGR is well-suited for this purpose, as most glutathione reductases described in the literature use NADPH instead of NADH as an electron donor (Reiter et al. 2014). When AvGR was not present in an assay in which GGE concentrations were greater than those of NAD⁺ and GSH, there was incomplete hydrolysis of this diaromatic substrate, accumulation of β-etherase pathway intermediates, and depletion of NAD⁺, as expected if the reaction was cofactor limited.

We were also able to detect the release of tricin when GTE was used as a substrate in this assay, showing for the first time that β-etherase pathway enzymes are capable of β-ether bond cleavage in a substrate bearing a large flavonoid moiety. This further shows that the β-etherase pathway enzymes are not limited to substrates containing only G and S monoaromatic units. In prior research, we had demonstrated the ability of LigE and LigF to cleave G-G, G-S, S-G, and S—S dimer models (Gall and Ralph et al. 2014, Gall and Kim et al. 2014), so this result extends the knowledge of the diversity of substrates for these enzymes to the G-T dimers. Thus, although the β-etherase pathway enzymes are thought to be highly stereospecific, they are also capable of recognizing the many different configurations of β-ether linked aromatics potentially present in lignin. With the results of these and previous findings combined (Gall and Ralph et al. 2014, Gall and Kim et al. 2014), we conclude that the minimal set of enzymes used in this study is sufficient to enable the β-etherase pathway in vitro to release of G, S, and T units from compounds modeling β-ether units in lignin.

This coupled assay also allowed us to directly compare the ability of LigG and NaGST_(Nu) to function in the β-etherase pathway. We found that the presence of NaGST_(Nu), AvGR, along with LigD, LigN, LigE, and LigF, was sufficient to allow complete conversion of GGE to HPV and guaiacol (FIG. 17A (panels A-C)). This is consistent with our prediction that NaGST_(Nu) can accommodate both GS-HPV epimers in its active site (Kontur et al. 2018) and the ability of this enzyme to produce stoichiometric amounts of HPV from GGE when added to this coupled assay. In contrast when LigG replaced NaGST_(Nu) under otherwise identical assay conditions, there was incomplete hydrolysis of GGE to HPV and guaiacol, with significant accumulation of GGE-ketone and lower amounts of GS-HPV (FIGS. 17A and 17B (panels A,D-E)). Thus, although it has been suggested that LigG can hydrolyze both β-epimers of GS-HPV (Rosini et al. 2016), this result, along with those published previously (Gall and Kim et al. 2014), support the hypothesis that LigG has a strong preference for β(R)-GS-HPV. This direct comparison of substrate conversion to products in assays that differ only in the addition of LigG or NaGST_(Nu) allows us to conclude that use of the latter enzyme has advantages owing to its greater ability to release HPV from both GS-HPV epimers under comparable conditions in vitro.

Release of Aromatic Monomers from Lignin Oligomers In Vitro.

The features of this coupled β-etherase assay allowed us to begin testing the ability to remove monomer aromatics from bona fide lignin. Lignin is a heterogeneous, high molecular weight polymer, with only limited solubility under the aqueous buffer conditions used for this assay. Consequently, to increase our chances of observing aromatic products under the conditions used for the coupled assay, we used several different lignin oligomers. We also fractionated these materials to test for release of aromatics from different sized lignin oligomers. This has provided several important new insights into the activity of β-etherase enzymes with lignin oligomers and identified opportunities for increasing our understanding of this pathway.

We tested the ability of this enzyme mixture to cleave lignin oligomers that were derived from an engineered poplar line that contains a high content of S units (Stewart et al. 2009). HPS was detected as a product when high molecular weight fractions of the HP lignin were used as the substrate. This provides direct proof that the enzyme mixture will cleave aromatic oligomers containing S units and that this set of β-etherase pathway enzymes are active with lignin oligomers. Given that the vast majority of the aromatic units in HP lignin are S units (Stewart et al. 2009), we estimate that the oligomers used in the enzymatic assay had between 40 and 50 aromatic units (Table 9). With the concentration of lignin oligomer used in this assay (˜2.2 mg mL⁻¹), complete substrate degradation would yield ˜8 mM HPS. The measured HPS concentration in this assay was 1.0 mM, resulting in a 12.5% yield of HPS from HP lignin. Thus, it appears that the mixture of enzymes used in this study, although sufficient for complete cleavage of model diaromatic compounds, and of some β-ether links in HP lignin, is not capable of complete cleavage of all the β-ether linkages in the HP lignin oligomers. It is possible that a heretofore undescribed protein is required to further process these lignin oligomers, or that inhibition of enzyme activity was caused by the presence of some of the high MW oligomers. Although our findings with the model dimers, and previous research, indicate that LigD and LigN are sufficient for complete oxidation of diaromatic compounds (FIGS. 17A and 17B) (Sato et al. 2009, Hishiyama et al. 2012), it is possible that the seemingly redundant dehydrogenases LigO and LigL have a higher affinity for higher MW lignin oligomers. Similarly, LigP, a GSH-S-transferase with apparent redundant activity with LigE (Tanamura et al. 2010), may be of interest for the optimization of in vitro lignin depolymerization.

In the assays using HP lignin as a substrate, we did not detect syringaresinol as a product, even though this dimer is found in low abundance in this polymer (Stewart et al. 2009). Existing models for the composition of HP lignin predict that syringaresinol is primarily internal to the polymer (Stewart et al. 2009). Thus, it is possible that the failure to detect syringaresinol as a reaction product reflects the inability of the tested β-etherase enzymes to access and cleave β-ether bonds that are adjacent to a syringaresinol moiety or that the enzymes exhibited only limited exolytic activity, thus never reaching the syringaresinol unit.

Having established that the coupled enzymatic assay exhibited β-etherase catalytic activity with high-MW fractions of the HP lignin oligomers, we tested a more complex lignin sample from corn stover as a substrate (MCS lignin). Fractionation of this lignin was also carried out and experiments with a wider array of lignin fractions were conducted to test for the release of the major aromatic monomers present in this material (G, S and T units). The detection of HPV, HPS, and tricin from different MCS lignin fractions confirms the observations with the β-ether linked models that the enzyme set used was active in the release of G, S, and T units from lignin. However, tricin was only observed with the lignin fraction having an average MW of 460 (FIG. 22). Using a crude assumption that the average aromatic unit in lignin has a MW of 210 and the known MW of tricin (330), this fraction represents mostly lignin dimers or a T unit with at most one or two other S or G unit. Thus, the ability of the enzymes to cleave the β-ether linkage next to a flavonoid moiety appears restricted to lower MW oligomers. In contrast, HPS and HPV were released from MCS lignin in assays using all of the fractions tested (FIGS. 23A and 23B), which we estimate to encompass a range of oligomers from dimers to 50-unit oligomers (Table 10). The highest measured concentration of HPS and HPV corresponded to the lignin fraction with average MW of 1,390, or ˜7 aromatic units (Table 10). Using the same assumption of 210 as the average MW of an aromatic unit in lignin, and the mass of lignin used in the assay (2.2 mg mL⁻¹), we estimate a yield of HPS plus HPV of ˜5%, which is lower than the estimated HPS yield from HP lignin. This lower release of substrates from MCS than HP lignin likely reflects the more heterogeneous and complex structure of the MCS lignin sample and potential inability of the β-etherase pathway enzymes to access and cleave all β-ether bonds in the polymer.

Taken together, the findings presented here reveal new and exciting features of the β-etherase pathway enzymes. We identified tricin as a valuable flavonoid that can be enzymatically cleaved from β-ether linked models and from low-MW lignin fractions. We also demonstrated β-etherase activity with intact lignin oligomers of varying sizes, some of which might even be too large to be transported into cells. These findings therefore provide the first demonstration that in vitro depolymerization of lignin is possible with β-etherase enzymes, an important step towards the development of biotechnological applications designed to derive high-value monomeric compounds from bona fide lignin polymers. The activity of this set of enzymes on oligomeric substrates provides an opportunity to develop and optimize conditions for aromatic release from lignin fractions derived from biomass deconstruction chemistries that are or will be used by industry.

Abbreviations

NAD⁺, nicotinamide adenine dinucleotide; NADH, reduced nicotinamide adenine di-nucleotide; GSH, glutathione; GSSG, glutathione disulfide; GS-HPV, β-S-glutathionyl-γ-hydroxypropiovanillone; GS-HPS, β-S -glutathionyl-γ-hydroxypropiosyringone; HPV, γ-hydroxypropiovanillone; HPS, γ-hydroxypropiosyringone; GGE, guaiacylglycerol-β-guaiacyl ether; GGE-ketone, α-oxidized GGE; GTE, guaiacylglycerol-β-tricin ether; GTE-ketone, α-oxidized GTE; NaGST_(Nu), Novosphingobium aromaticivorans strain DSM12444 glutathione lyase; AvGR, Allochromatium vinosum DSM180 glutathione reductase; GPC, gel-permeation chromatography.

Example 3 A Heterodimeric β-Etherase Capable of Sterospecifically Breaking the β-Aryl Ether Bond Commonly Found in Lignin Summary

This example describes a newly identified enzyme that can cleave the major β-aryl ether linkage in plant lignin. Lignin is a heterogeneous polymer of aromatic units that can constitute as much as 30% of a plant's dry cell weight, making it one of the most abundant renewable materials on Earth. Currently, there are few economical uses for lignin; the polymer is typically disposed of or burned for energy. The aromatic compounds that make up lignin could potentially be used in the chemical, cosmetic, food, and pharmaceutical industries; however, due largely to its irregular, covalently bonded structure, lignin has historically been difficult to depolymerize. Consequently, intensive efforts are currently aimed at developing chemical, enzymatic, and hybrid methods for deriving simpler and lower molecular weight products from lignin.

Some sphingomonad bacteria (e.g. Novosphingobium aromaticivorans) can break the bonds between aromatic units in the lignin polymer, including the β-aryl ether ((3-O-4) bond, the most common linkage between aromatic units in lignin (typically >50% of the total linkages). The sphingomonad pathway for breaking the β-aryl ether bond involves three initial steps. First, the α-hydroxyl is oxidized by one of several stereospecific NAD⁺-dependent dehydrogenases (LigL, LigN, LigD, LigO). Next, stereospecific β-etherases (LigF, LigE, LigP) replace the β-ether bond of the resulting α-ketone with a thioether bond involving glutathione (GSH), releasing a glutathione conjugated phenylpropanoid. Finally, the glutathione moiety is removed from the GS-phenylpropanoid by either a stereospecific (i.e. LigG) or non-stereospecific (i.e. GST_(Nu)) glutathione lyase. All of the characterized GSH-dependent β-etherases in this pathway function as homodimers.

The β-etherases that react with a particular stereoisomer of the β-aryl ether bond are similar in amino acid sequence to each other. These β-aryl etherases fall into distinct groups that cleave either the R- (LigE and LigP homodimers) or the S-stereoisomers (LigF homodimers), and the enzymes that cleave the two different stereoisomers of the β-aryl ether bond are phylogenetically distinct from each other. We report here a heterodimeric β-aryl etherase (BaeA, comprised of the Saro_2872 and Saro_2873 proteins) that cleaves the R-stereoisomer of the β-aryl ether bond (like LigE and LigP), but is composed of polypeptides that are more similar in sequence to (but still phylogenetically distinct from) the enzymes in the LigF group.

This expands the known range of enzymes capable of breaking the β-aryl ether bond commonly found in lignin, some of which may have kinetic or other properties better suited to operating within an in vitro lignin depolymerization system than the previously characterized LigE and LigP enzymes.

Construction of N. Aromaticivorans Mutants

Biological Reagents.

All PCR reactions were performed with Herculase II polymerase (Agilent Technologies, Santa Clara, Calif.). Primers were phosphorylated with polynucleotide kinase from Promega (Madison, Wis.). All other enzymes were from New England Biolabs (Ipswich, Mass.). All primers were from Integrated DNA Technologies (Coralville, Iowa).

For cloning using the NEBuilder HiFi Assembly system (New England Biolabs), plasmid pK18msB-MCS (Schafer et al. 1994) was amplified using primers “pK18msB AseI ampl F” and “pK18msB-MCS XbaI R” to generate the linear fragment pK18msB-MCS (see Example 1 above).

Strains.

The strains used in the present example are presented in Table 11.

TABLE 1 Novosphingobium aromaticivorans strains used in this example. Strain Genotype Reference 12444Δ1879 DSM 12444 ΔSaro_1879 Examples above 12444ΔligE 1244441879 ΔSaro_2405 This example 12444Δ2872 1244441879 ΔSaro_2872 This example 12444Δ2873 1244441879 ΔSaro_2873 This example 12444ΔligEΔ2872 1244441879 ΔSaro_2405 ΔSaro_2872 This example

TABLE 12 Primers used in genomic modifications and enzyme expression. Name Sequence Notes pK18msB 5′-CTGTCGTGCCAGCTGCATTAATG-3′ (SEQ ID AseI site AseI ampl F NO: 64) (underlined) native to template pK18msB- 5′-GAACAtcTAGAAAGCCAGTCCGCAGAA AC- XbaI site MCS XbaI R 3′ (SEQ ID NO: 65) (underlined); lowercase bases do not match template pK18-ligE 5′-GTTTCTGCGGACTGGCTTTCTAGATGTTCC Underlined region OvExt F AGTGCTCTACAACCAGTCGTACCACATG-3′ is complementary (SEQ ID NO: 97) to pK18msB-MCS pK18-ligE 5′-CGATTCATTAATGCAGCTGGCACGACAGCG Underlined region OvExt R AGTTGAACGAAACCTCCTCGTTCATG-3′ (SEQ is complementary ID NO: 98) to pK18msB-MCS Saro2405 5′-GCATCACCGAAGGCATGAAGAAGTAAACG- ligE del F 3′ (SEQ ID NO: 99) Saro2405 5′-GTGACTCAATTGCCGTCACCCTGAACTTG-3′ ligE del R (SEQ ID NO: 100) Saro_2872 5′-CATCattaATTCGACCTGGCCATAGGACTG-3′ AseI site ampl AseI F2 (SEQ ID NO: 101) (underlined); lowercase bases do not match template Saro_2872 5′-taGttCtaGACCATCTTTTCCGCTGGAGC-3′ XbaI site ampl XbaI R (SEQ ID NO: 102) (underlined); lowercase bases do not match template Saro_2872 5′-GCTTGTCAAGGCCTGGCTTGC-3′ (SEQ ID del R NO: 103) Saro_2872 5′-TtATCCCTCGATCTCCGCCATGATGAG-3′ lowercase base del F (SEQ ID NO: 104) does not match template Saro_2873- 5′-GTTTCTGCGGACTGGCTTTCTAGATGTTCCC Underlined region pk18 hifi TACAAGGGAGGGCAGTGAAATGAAGC-3′ (SEQ is complementary ampl R ID NO: 105) to pK18msB-MCS Saro_2873 5′-CATCCCTCGATCTCGTCCATCCGCTGCCCA Underlined region hifi del F TCC-3′ (SEQ ID NO: 106) is complementary to Saro_2873 hifi del R Saro_2873- 5′-CGATTCATTAATGCAGCTGGCACGACAGG Underlined region pk18 hifi GACGAATGATAGACCAGCCACTTCAGG-3′ is complementary ampl F (SEQ ID NO: 107) to pK18msB-MCS Saro_2873 5′-GATGGACGAGATCGAGGGATGAGCGCGCT Underlined region hifi del R TCTTTACC-3′ (SEQ ID NO: 108) is complementary to Saro_2873 hifi del F Saro2872 5′-GGCatctgcgaGaccTCCCCAACGGTTGATTTC BsaI site Ctag BsaI F AG-3′ (SEQ ID NO: 109) (underlined); lowercase bases do not match template Saro2872 5′-CGAGtcATGAGCGCGCTTCTTTACCACG-3′ BspHI site Ctag BspHI R (SEQ ID NO: 110) (underlined); lowercase bases do not match template pVP302K 5′-CTGCGGTCTCGCAGATGGTAAAATTCTG-3′ BsaI site Ctag BsaI F (SEQ ID NO: 80) (underlined) pVP302K 5′-GGTGATGTCCCATGGTTAATTTCTCCTCTTT NcoI site Ctag NcoI R AATG-3′ (SEQ ID NO: 81) (underlined) Ctag 2872- 5′-CGAGttaTCCCCAACGGTTGATTTCAGG-3′ lowercase bases do pVP add (SEQ ID NO: 111) not match template Stop R pVP302K 5′-CATTAAaAGcTTAAACGAATTCGGACTCGG HindIII site Ntag HindIII F TACGC-3′ (SEQ ID NO: 83) (underlined); lowercase bases do not match template 2872-pVP C 5′-caagcgaaaatctgtattttcagagcgcgatcgcaggaATGAGC lowercase bases do to Ntag F GCGCTTCTTTACCACG-3′ (SEQ ID NO: 112) not match template pVP302 C to 5′-ccaatgcatggtgatggtgatgatggtgatgtcccatGGTTAATT lowercase bases do Ntag R TCTCCTCTTTAATG-3′ (SEQ ID NO: 85) not match template Saro2872 5′-caagcgaaaatctgtattttcagagcgcgatcgcaggaAGCGCG lowercase bases do gNtag R CTTCTTTACCACGG-3′ (SEQ ID NO: 113) not match template Saro2872 5′-ccaatgcatggtgatggtgatgatggtgatgtaTCATCCCTCG lowercase bases do gNtag F ATCTCCGCCATGATG-3′ (SEQ ID NO: 114) not match template 2872- 5′-CTAACTTTGTTATTTTCGGCTTTCTGTTATC Underlined region 3_pVP_HiFi_F CCCAACGGTTGATTTCAGG-3′ (SEQ ID NO: 115) is complementary to pVP302K Saro2872- 5′-GAATTCATTAAAGAGGAGAAATTAACCAT Underlined region 3NOTAG_pVP_HiFi_R GGACGAGGTAAGCCTCTATCATTGG-3′ (SEQ ID is complementary NO: 116) to pVP302K pVP302K- 5′-GGTTAATTTCTCCTCTTTAATGAATTCTGTG HiFi-noTag-R TGAAATTG-3′ (SEQ ID NO: 117) pVP302K- 5′-CAGAAAGCCGAAAATAACAAAGTTAGCCT HiFi-ATW-F GAGCTG-3′ (SEQ ID NO: 91) Saro2872- 5′-GTATTTTCAGAGCGCGATCGCAGGAATGG Underlined region 3Ntag_pVP_HiFi_R ACGAGGTAAGCCTCTATCATTGG-3′ (SEQ ID is complementary NO: 118) to pVP302K pVP302K- 5′-TCCTGCGATCGCGCTCTGAAAATACAGATT HiFi-ATW-R TTCG-3′ (SEQ ID NO: 90) Saro2872- 5′-CGCGgCGCTCACCGTTCTTGC-3′ (SEQ ID Lowercase g S14A_R NO: 119) introduces S→A mutation in underlined codon Saro2872- 5′-CCGTTGGGCTCGCCGTGGTAAAGAAG-3′ S14A_F (SEQ ID NO: 120) Saro2873- 5′-GCAAGCCGATGCTCGCGTTGATG-3′ (SEQ ID S15A_R NO: 121) Saro2873- 5′-CAGcGTTGGCATTGGGTTCCCAATGATAG Lowercase c S15A_F AG-3′ (SEQ ID NO: 122) introduces S→A mutation in underlined codon Saro2873- 5′-CAGAGgcGGCATTGGGTTCCCAATGATAG Lowercase gc N14A_F AG-3′ (SEQ ID NO: 123) introduces N→A mutation in underlined codon pEU-HiFi- 5′-GTGATGATGATGATGATGTCCCATTAAC-3′ ATW-R (SEQ ID NO: 124) pEU-HiFi- 5′-TAGTTTAAACGAATTCGAGCTCGG-3′ (SEQ ATW-F ID NO: 125) Saro2872- 5′-GGACATCATCATCATCATCACGCATTGGCA Underlined region pEU2394- AGCGAAAATCTGTATTTTCAG-3′ (SEQ ID is complementary HiFi-F NO: 126) to pEU Saro2872- 5′-CCGAGCTCGAATTCGTTTAAACTACGAGTT Underlined region pEU2394- ATCCCCAACGGTTGATTTCAGG-3′ (SEQ ID is complementary HiFi-R NO: 127) to pEU pEU-2872- 5′-CATTAACTAACTAGTGTAGTTGTAGAATGT Underlined region fix-R AAAATGTAATGTTGTTGTTGTTTG-3′ (SEQ ID was missing in NO: 128) originally created pEU-2872 pEU-2872- 5′-GGACATCATCATCATCATCACGCATTGG-3′ fix-F (SEQ ID NO: 129) Saro_2873- 5′-CAACTACACTAGTTAGTTAATGGACGAGGT Underlined region pEU_HiFi-F AAGCCTCTATCATTGG-3′ (SEQ ID NO: 130) is complementary to pEU Saro_2873- 5′-CGAGCTCGAATTCGTTTAAACTACTCATCC Underlined region pEU_HiFi-R CTCGATCTCCGCCATG-3′ (SEQ ID NO: 131) is complementary to pEU pEU2394 F 5′-GTAGTTTAAACGAATTCGAGCTCGGTACC-3′ (SEQ ID NO: 132)

Plasmid for Deleting Saro_2405.

A 3844 bp region of the N. aromaticivorans genome extending from 1501 bp upstream of Saro_2405 to 1503 bp downstream of the gene was amplified from purified genomic DNA using primers “pK18-ligE OvExt F” and “pK18-ligE OvExt R”, which contain 5′ ends that are complementary to the ends of linearized pK18msB-MCS. The genomic DNA fragment was combined with linearized pK18msB-MCS using the NEBuilder HiFi Assembly system to produce plasmid pK18msB-ligE. This plasmid was amplified using kinase phosphorylated primers “Saro2405 ligE del F” and “Saro2405 ligE del R” to produce a linear fragment in which the majority of Saro_2405 (including the start codon) was missing. This linear fragment was circularized using T4 DNA Ligase to generate plasmid pK18msB-ΔligE.

Plasmid for Deleting Saro_2872.

A ˜2813 bp region of the N. aromaticivorans genome extending from 1073 bp upstream of Saro_2872 to 954 bp downstream of the gene was amplified from purified genomic DNA using primers “Saro_2872 ampl AseI F2” and “Saro_2872 ampl XbaI R”, which contain recognition sites for the restriction enzymes AseI or XbaI, respectively, incorporated into their 5′ ends. The resulting fragment was digested with AseI and XbaI, then ligated with pK18msB-MCS that had been digested with AseI and XbaI, using T4 DNA Ligase, to form plasmid pK18msB-Saro2872. This plasmid was amplified using kinase phosphorylated primers “Saro_2872 del R” and “Saro_2872 del F” to produce a linear fragment in which the majority of Saro_2872 was missing. Since the start codon of Saro_2872 overlaps with the stop codon of Saro_2873, “Saro_2872 del F” contains a single base mismatch with pK18msB-Saro2872, to inactivate the Saro_2872 start codon, while preserving the Saro_2873 stop codon. This linear fragment was circularized using T4 DNA Ligase to generate plasmid pK18msB-ΔSaro2872.

Plasmid for Deleting Saro_2873.

˜1100 bp regions from upstream and downstream of Saro_2873 in the N. aromaticivorans genome were separately amplified from purified genomic DNA using primer sets “Saro_2873-pk18 hifi ampl R” and “Saro_2873 hifi del F”, and “Saro_2873-pk18 hifi ampl F” and “Saro_2873 hifi del R”, respectively. These two fragments were combined with linearized pK18msB-MCS using the NEBuilder HiFi Assembly system to produce plasmid pK18msB-ΔSaro2873, in which the regions that naturally flank Saro_2873 in the genome are adjacent to each other.

Deleting Genes from the N. aromaticivorans Genome.

Deletion plasmids were separately mobilized into N. aromaticivorans via conjugation with Escherichia coli S17-1. For the conjugation, cultures of E. coli S17-1 harboring the plasmid and N. aromaticivorans were grown up overnight in Lysogeny Broth containing kanamycin or GluSis, respectively. Cultures were subcultured and allowed to resume exponential growth before being harvested by centrifugation. E. coli and N. aromaticivorans cell pellets were washed in lysogeny broth, then resuspended together into 90 μL lysogeny broth. Conjugations were allowed to proceed overnight at 30° C. The following day, the conjugations were outgrown in GluSis at 30° C. for >1 h, then plated onto solid GluSis with kanamycin to select for N. aromaticivorans cells in which the plasmid had incorporated into the genome via homologous recombination (single crossovers). Single crossovers were confirmed through the inability to immediately grow on GluSis containing 10% sucrose.

Single crossovers were cultured in 5 mL of GluSis containing 10% sucrose and shaken at 30° C. until growth commenced (usually after several days), which signified loss of the plasmid from the genome via a second round of homologous recombination. These cultures were streaked onto solid GluSis+10% sucrose to isolate individual strains that has lost the plasmid (double crossovers), and plasmid loss was confirmed by the inability to grow on GluSis+kanamycin. The absences of the desired genes were confirmed via PCR performed on isolated genomic DNA and Sanger sequencing.

Bacterial Growth Media

E. coli cultures used for cloning were grown in lysogeny broth (LB), and shaken at ˜200 rpm at 37° C. For routine storage and manipulation, N. aromaticivorans cultures were grown in LB or GluSis at 30° C. GluSis is a modification of Sistrom's minimal medium in which the succinate has been replaced by 22.6 mM glucose (see Example 1, above). N. aromaticivorans growth experiments used Standard Mineral Base (SMB) minimal medium, as described in Example 1, except at pH 7.0. Where needed to select for plasmids, media were supplemented with 100 μg/mL ampicillin, 50 μg/mL kanamycin, or 20 μg/mL chloramphenicol.

N. Aromaticivorans Growth Experiments

Starter cultures of N. aromaticivorans were grown in 4 mL SMB containing 4 mM vanillate. Experimental cultures were grown in 20-30 mL of SMB containing 3 mM vanillate and 1 mM GGE, in 125 mL conical growth flasks shaken at 200 rpm at 30° C. Aliquots (400-600 μL) were removed at specified time points and filtered through 0.22 μm syringe tip filters (e.g. Whatman Puradisc filters, GE Healthcare) before HPLC analysis of extracellular aromatics. Every culture was grown at least three times; data shown are from representative cultures.

For the 12444ΔligEΔ2872 and 12444ΔligEΔ2873 cultures, we filtered >2 mL for the final time points. These samples still contained MPHPV; to determine which stereoisomer(s) of MPHPV remained present, the samples were split into three 400 uL aliquots and combined with 5 mM GSH and either H₂O, recombinant LigE (90 μg/mL), or recombinant LigF1 (147 μg/mL), and incubated at 30° C. for 1 h. These samples were then analyzed via HPLC as described below.

Expression and Purification of Recombinant Proteins

Plasmid for Expressing Recombinant Saro_2872.

Saro_2872 was amplified from N. aromaticivorans genomic DNA with the primers “Saro2872 Ctag BsaI F” and “Saro2872 Ctag BspHI R”. This fragment was digested with restriction enzymes BspHI and BsaI. The expression vector pVP302K (Gall and Ralph et al. 2014) was amplified using the primers “pVP302K Ctag BsaI F” and “pVP302K Ctag NcoI R”, and the resulting fragment was digested with BsaI and NcoI. The digested fragments were ligated using T4 DNA ligase, generating plasmid pVP302K/Ctag-2872, which consists of a T5 promoter followed by the coding sequences of Saro_2872 (absent the stop codon), the RtxA protease from Vibrio cholerae, and a His₈ tag.

pVP302K/Ctag-2872 was amplified using kinase phosphorylated primers “Ctag 2872-pVP add Stop R” and “pVP302K Ntag HindIII F”. This fragment was circularized using T4 DNA ligase to generate plasmid pVP302K/Untagged2872, in which a stop codon has been introduced directly after Saro_2872.

pVP302K/Untagged2872 was amplified via PCR using kinase phosphorylated primers “2872-pVP C to Ntag F” and “pVP302 C to Ntag R”. The amplified fragment was circularized using T4 DNA ligase to generate plasmid pVP302K/Ntag-2872, which contains a T5 promoter followed by coding sequences for a His₈-tag, a tobacco etch virus (Tev) protease recognition site and Saro_2872.

Plasmids for Expressing Recombinant Saro_2872 and Saro_2873 Together (BaeA).

To Express BaeA Containing a His₈-Tag on the N-Terminus of Saro_2872:

We first generated a strain of N. aromaticivorans in which a coding sequence for a His₈-tag was incorporated into the genome so that cellular copies of Saro_2872 protein would contain a His₈-tag on their N-terminus. pK18msB-Saro2872 was amplified via PCR using kinase phosphorylated primers “Saro2872 gNtag R” and “Saro2872 gNtag F”, to generate a fragment containing Saro_2873 (with its stop codon), followed by a coding sequence for a His₈-tag, then a Tev protease recognition site, then Saro_2872 (missing its native start codon). This fragment was circularized using T4 DNA ligase to generate plasmid pK18msB-H₈Saro2872. pK18msB-H₈Saro2872 was mobilized into strain 1244442872 via conjugation from E. coli S17-1, and a strain of N. aromaticivorans (12444-H₈2872) containing the coding sequence for Saro_2872 containing an N-terminal His₈-tag was generated and isolated using homologous recombination as described above for generating deletion mutants.

We ran PCR using genomic DNA from strain 12444-H₈2872 as template with primers “2872-3_pVP_HiFi_F” and “Saro2872-3NOTAG_pVP_HiFi_R” to generate a fragment containing the coding sequence for Saro_2873 (with stop codon intact), followed by the coding sequence for a His₈-tag, then for Saro_2872 (missing its start codon), with extensions on the ends of the fragment that are complementary to plasmid pVP302K. pVP302K was amplified via PCR using the primers “pVP302K-HiFi-noTag-R” and “pVP302K-HiFi-ATW-F”. These two fragments were combined using the NEBuilder HiFi Assembly system to create plasmid pVP302K/2873-H2872.

To Express BaeA Containing a His₈-Tag on the N-Terminus of Saro_2873:

We ran PCR using genomic DNA from strain 12444Δ1879 as template with primers “2872-3_pVP_HiFi_F” and “Saro2872-3Ntag_pVP_HiFi_R” to generate a fragment containing the native genomic organization of the Saro_2873 and Saro_2872 genes, with extensions on the ends of the fragment that are complementary to plasmid pVP302K. pVP302K was amplified via PCR using the primers “pVP302K-HiFi-ATW-R” and “pVP302K-HiFi-ATW-F”. These two fragments were combined using the NEBuilder HiFi Assembly system to create plasmid pVP302K/H2873-2872.

To Express BaeA Mutants:

To generate mutant 2:S14A, pVP302K/2873-H2872 was amplified by kinase phosphorylated primers “Saro2872-S14A_R” and “Saro2872-S14A_F”. To generate mutant 3:S15A, pVP302K/H2873-2872 was amplified by kinase phosphorylated primers “Saro2873-S15A_R” and “Saro2873-S15A_F”. To generate mutant 3:N14A, pVP302K/2873-H2872 was amplified by kinase phosphorylated primers “Saro2873-S15A_R” and “Saro2873-N14A_F”. These linear fragments were separately circularized using T4 DNA ligase to generate plasmids pVP302K/2873-H2872(S14A), pVP302K/H2873(S15A)-2872, and pVP302K/2873(N14A)-H2872, respectively.

To generate mutant 2:14A/3:S15A, pVP302K/2873-H2872(S14A) was amplified by kinase phosphorylated primers “Saro2873-S15A_R” and “Saro2873-S15A_F”. The linear fragment was circularized using T4 DNA ligase to generate plasmid pVP302K/2873(S15A)-H2872(S14A).

Plasmids for Expressing Recombinant Saro_2873.

We amplified plasmids pVP302K/2873-H2872 and pVP302K/H2873-2872 via PCR using kinase phosphorylated primers “pVP302K-HiFi-ATW-F” and “Saro_2872 del F”. These linear fragments were separately circularized using T4 DNA ligase to generate plasmids pVP302K/Untagged2873 and pVP302K/Ntag-2873, respectively.

Expression and Purification of Recombinant Enzymes.

Recombinant proteins were expressed using the plasmids described above in E. coli B834 containing plasmid pRARE2 (Novagen) grown for ˜25 hours at 25° C. in ZYM-5052 Autoinduction Medium containing kanamycin and chloramphenicol. Recombinant proteins were purified using a Ni²⁺-NTA column as described in Example 1 above, except using gravity-flow columns instead of an FPLC system. After removal of His₈-tags using Tev protease, recombinant proteins retained a Ser-Ala-Ile-Ala-Gly-peptide on their N-termini, derived from the linker between the protein and the Tev protease recognition site. Recombinant LigF1 was purified as previously described (Gall and Ralph et al. 2014).

Recombinant enzyme concentrations were determined via the Bradford method (absorbance at 595 nm), using known concentrations of bovine serum albumin as standards (Thermo Scientific) and protein assay dye reagent from Biorad.

Cell-Free Synthesis of Saro_2872 and Saro_2873

Plasmid for Expressing Saro_2872 in a Cell-Free System.

Plasmid pEU-NGFP (Goren et al. 2009) was amplified via PCR using primers “pEU-HiFi-ATW-R” and “pEU-HiFi-ATW-F”to generate a linear fragment in which the gene for Green Fluorescent Protein has been removed. pVP302K/Ntag-2872 was amplified via PCR using primers “Saro2872-pEU2394-HiFi-F” and “Saro2872-pEU2394-HiFi-R” to generate a linear fragment containing the coding sequence for the Tev protease recognition site followed by Saro_2872. These linear fragments were combined using the NEBuilder HiFi Assembly system to create a plasmid that was missing a short sequence upstream of the translational start site. To add this sequence, we amplified the plasmid using kinase phosphorylated primers “pEU-2872-fix-R” and “pEU-2872-fix-F”. The linear fragment was circularized using T4 DNA ligase to form plasmid pEU-H2872, which contains a sequence for a His₆-tag, followed by a Tev protease recognition site, then Saro_2872.

Plasmid for Expressing Saro_2873 in a Cell-Free System.

N. aromaticivorans genomic DNA was amplified via PCR using primers “Saro_2873-pEU_HiFi-F” and “Saro_2873-pEU_HiFi-R” to generate a linear fragment containing Saro_2873 with ends that are complementary to pEU. pEU-H2872 was amplified via PCR using primers “pEU-2872-fix-R” and “pEU2394 F” to generate a linear fragment in which the sequences for the His₆-tag, the Tev protease recognition site, and Saro_2872 were removed. These linear fragments were combined using the NEBuilder HiFi Assembly system to create plasmid pEU-2873.

Cell-Free Protein Synthesis.

Cell-free protein synthesis was run essentially as previously described (Makino et al. 2014). The Saro_2872 polypeptide contained a His₆-tag and a Tev protease recognition site on its N-terminus that were not removed. The Saro_2873 polypeptide was synthesized in its native form. Synthesized polypeptides were not purified from the synthesis reaction mixture; assays for enzymatic activity were performed by adding aliquots directly from the synthesis reaction. Concentrations of the Saro_2872 and Saro_2873 polypeptides in the reaction mixtures were approximated using the intensities of the bands in an SDS-PAGE gel.

Assays to Determine Activities and Stereospecificties of Saro_2872 and Saro_2873

0.1 mM racemic (β(S) and β(R)) MPHPV was combined with 5.8 mM glutathione (GSH) in reaction buffer (RB; 25 mM Tris-HCl (pH 8.0) and 25 mM NaCl). Cell-free protein synthesis mixtures containing Saro_2872 and Saro_2873 were added individually or together to the MPHPV/GSH solutions to achieve concentrations of ˜24 nM of each polypeptide. These 625 μL reactions were incubated at 30° C. for 24 h to several days. Each was then split into 190 μL aliquots and combined with an additional 2.3 mM GSH and either H₂O, 151 μg/mL LigE (Saro_2405), or 184 μg/mL LigF1 (Saro_2091). These 212 μL reactions were incubated at 30° C. for several hours, then analyzed via HPLC.

Kinetics of the Enzymatic Cleavage of β(R)-MPHPV.

Various concentrations of racemic MPHPV (equal amounts of the β(S)- and β(R)-stereoisomers) were combined with 5 mM GSH in RB. At time zero, 100 μL of a given enzyme in RB+5 mM GSH was combined with 1000 μL of the racemic MPHPV/GSH sample at 25° C. (both samples were equilibrated to 25° C. before mixing). Final concentrations of β(R)-MPHPV in each reaction were 0.0045, 0.010, 0.017, 0.068, or 0.13 mM. Final enzyme concentrations were 18 nM BaeA, 23 nM BaeA (2:S14A), 22 nM BaeA (3:S15A), 24 nM BaeA (2:S14A/3:S15A), 98 nM BaeA (3N14A), or 70 nM LigE (Saro_2405) (all concentrations are for the dimeric enzyme, except for LigE, which is the concentration of the monomer). At specified time points, 200 μL of a reaction was removed and combined with 40 μL of 1 M HCl (Acros Organics) to stop the reaction before HPLC analysis to quantify GS-HPV formed. Control experiments were allowed to proceed for several hours to ensure that only the β(R)-MPHPV in the reaction mixtures was being reacted with in these experiments.

HPLC Analysis.

Analysis and quantification of aromatic compounds were performed using an Ultra AQ C18 5 μm column (Restek) attached to a System Gold HPLC (Beckman Coulter) with running buffers and methods described in Example 1. The eluent was analyzed for light absorbance between 191 and 600 nm, and absorbances at 280 nm were used for quantification of aromatic metabolites by comparing peak areas to those of standards.

Results

An N. aromaticivorans Saro_2405 (ligE) Deletion Mutant can Completely Metabolize Erythro-GGE.

LigE from N. aromaticivorans is capable of stereospecifically breaking the β-aryl ether bond of the β(R) stereoisomers of MPHPV and other di-aromatic compounds in vitro. To investigate the in vivo role of LigE in N. aromaticivorans, we constructed a strain in which the gene for LigE (Saro_2405) was deleted from the genome (12444ΔligE), and grew it, along with its parent strain (12444Δ1879), in a medium containing vanillate and erythro-GGE.

As expected from the examples provided above, 12444Δ1879 completely consumed both the vanillate and the GGE. Metabolism of GGE proceeded through several intermediates, including both β(R) and β(S) MPHPV, which transiently appeared in the medium, then were taken back up by the cells (FIG. 24, panel A). Consistent with our examples above, only a trace amount of guaiacol appeared in the extracellular medium, and the glutathione conjugate GS-HPV was never observed in the medium.

As the gene product of Saro_2405 is the only predicted homologue of LigE and LigP in N. aromaticivorans, and LigE and LigP are the only sphingomonad enzymes known to be capable of breaking the β(R) stereoisomer of the β-aryl ether bond, we expected that strain 12444ΔligE would be incapable of fully metabolizing erythro-GGE. However, although MPHPV consistently disappeared from the medium slower for 12444ΔligE than for 12444Δ1879, 12444ΔligE was capable of completely removing racemic MPHPV from the medium (FIG. 24, panel B). As MPHPV disappeared from the medium, HPV accumulated in the medium up to a concentration roughly equal to the initial erythro-GGE concentration, suggesting that essentially all of the GGE was metabolized through MPHPV and into HPV. These results suggest that 12444ΔligE contains an enzyme capable of breaking the β-aryl ether bond of β(R)-MPHPV.

Saro_2872 and Saro_2873 are Required for Cleavage of β(R)-MPHPV in N. Aromaticivorans 12444ΔligE.

As LigE, LigP, and LigF are all classified as glutathione S-transferases, we expected that a glutathione S-transferase was reacting with β(R)-MPHPV in the 12444ΔligE strain. We thus investigated Saro_2872 and Saro_2873, which are annotated as coding for glutathione-S-transferases and are located in a gene cluster with Saro_2865, which codes for one of the two LigF isoforms in N. aromaticivorans. We separately deleted Saro_2872 and Saro_2873 from the genome of 12444ΔligE and found that neither of the resulting strains, 12444ΔligEΔ2872 and 12444ΔligEΔ2873, respectively, could fully metabolize erythro-GGE (FIG. 24, panels D and E). Each strain accumulated MPHPV in its medium to a concentration roughly one-half of the medium's initial erythro-GGE concentration, suggesting that they were deficient in metabolizing MPHPV.

Our method of analysis does not distinguish between the β(R) and β(S) stereoisomers of MPHPV. Therefore, to determine which stereoisomer(s) of MPHPV remained unreacted in the media of the 12444ΔligEΔ2872 and 12444ΔligEΔ2873 cultures, the spent media were filtered, and individual aliquots were combined with H₂O, or recombinant LigF or LigE, which are known to react stereospecifically with the β(S) or β(R) isomers of MPHPV, respectively (FIG. 25, panels F-K). Addition of LigF to the spent media samples resulted in the conversion of a small amount (<10%) of MPHPV into GS-HPV and guaiacol (FIG. 25, panels G,J), suggesting that some of the unconsumed MPHPV was the β(S) isomer. However, addition of LigE resulted in conversion of most of the MPHPV into GS-HPV and guaiacol (FIG. 25, panels H,K), suggesting that a large fraction of the MPHPV was the β(R) isomer. These results suggest that 12444ΔligE requires both Saro_2872 and Saro_2873 for complete metabolism of MPHPV, particularly the β(R) isomer.

To determine whether both Saro_2872 and Saro_2873 are necessary for complete metabolism of MPHPV in an N. aromaticivorans strain with a functional LigE, we deleted Saro_2872 from 12444Δ1879. The resulting strain (12444Δ2872) was capable of fully metabolizing erythro-GGE (FIG. 24, panel C), though it removed the MPHPV from the medium slower than 12444Δ1879 (FIG. 24, panel A), similar to 12444ΔligE (FIG. 24, panel B). Thus, it appears that either LigE or a combination of Saro_2872 and Saro_2873 is sufficient for completely metabolizing β(R)-MPHPV.

The Saro_2872 and Saro_2873 Polypeptides Form a Heterodimer that is Stereospecific for β(R)-MPHPV.

As our genetic results suggested that the Saro_2872 and Saro_2873 gene products contribute to cleavage of β(R)-MPHPV in N. aromaticivorans, we sought to express these proteins and test them for this activity in vitro.

Initial attempts to individually express and purify the Saro_2872 and Saro_2873 polypeptides recombinantly in Escherichia coli were unsuccessful. We thus separately expressed each polypeptide using a cell-free protein synthesis system. When the polypeptides were individually combined with racemic (β(R) and β(S)) MPHPV, a trace amount of GS-HPV appeared in the reactions (<1% of the initial MPHPV concentration), but essentially all of the MPHPV remained unreacted (FIG. 26, panels A-C), even after several days.

The lack of activity of the Saro_2872 and Saro_2873 polypeptides, and our observation that the Saro_2872 and Saro_2873 ORFs overlap in the N. aromaticivorans genome, led us to hypothesize that the polypeptides may form a heterodimer. Indeed, when we combined the separately prepared Saro_2872 and Saro_2873 polypeptides with each other and with racemic MPHPV, half of the MPHPV was converted into GS-HPV and guaiacol (FIG. 26, panel D). To determine which stereoisomer(s) of MPHPV remained unreacted in this reaction, we split the reaction mixture and added recombinant LigE (Saro_2405) or LigF1 (Saro_2091). Upon addition of LigE, no change in the amounts of MPHPV, GS-HPV, or guaiacol was observed (FIG. 26, panel E). Upon addition of LigF1, the remaining MPHPV in the reaction mixture was converted into GS-HPV and guaiacol (FIG. 26, panel F), suggesting that the MPHPV remaining after the reaction of racemic MPHPV with the mixture of Saro_2872 and Saro_2873 was the β(S) stereoisomer.

To generate larger amounts of the Saro_2872-Saro_2873 complex than was possible with the cell-free system, we attempted to express the Saro_2872 and Saro_2873 polypeptides together from a single expression vector in E. coli. Despite the fact that only one of the polypeptides contained a His₈-tag on its N-terminus, two polypeptides from the E. coli cell lysate, corresponding to the expected sizes of Saro_2872 and Saro_2873, reversibly bound to a Ni²⁺-NTA column, consistent with Saro_2872 and Saro_2873 forming a heterodimer. Indeed, the purified recombinant protein ran as a single peak that corresponded to a dimer in gel permeation chromatography experiments (FIG. 27).

In reactions similar to those performed with the cell-free generated polypeptides, we found that the recombinantly generated Saro_2872-Saro_2873 complex reacted specifically with β(R)-MPHPV, and did not react with β(S)-MPHPV. Because the Saro_2872-Saro_2873 heterodimer is a β-aryl etherase, we call the heterodimer BaeA. The fact that BaeA has the same stereospecificity as LigE (for β(R)-MPHPV) is curious, as both Saro_2872 and Saro_2873 cluster much closer to the previously characterized LigF enzymes than to the previously characterized LigE enzymes in a phylogenetic analysis (FIG. 28). This finding likely has implications for the evolution of the enzymatic ability to break the two stereoisomers of the β-aryl ether bond of lignin.

The Saro_2873 Subunit is Much More Catalytically Active than the Saro_2872 Subunit in BaeA.

We sought to gain insight into the relative activities of the Saro_2872 and Saro_2873 subunits in BaeA by independently inactivating one or the other of the subunits. Previous work found that LigF from Sphingobium sp. SYK-6 (SLG_08650) contains a serine residue in its active site (Ser¹⁴) that is important for reacting with β(S)-(1′-formyl-3′-methoxyphenoxy)-γ-hydroxypropioveratrone (an analogue of MPHPV): mutation of the serine had a dramatic effect on the reaction rate, although it was unclear whether the effect was from changes in substrate binding, turnover, or both (Helmich et al. 2016). This serine residue is conserved in all the previously characterized LigF enzymes, and in both Saro_2872 (Ser¹⁴) and Saro_2873 (Ser¹⁵) (FIG. 29). We mutated these serines into alanines separately (2:S14A and 3:S15A) and together (2:S14A/3:S15A) in BaeA and assayed the variant enzymes in vitro, along with wild-type BaeA. As we did not know the relative activities of the two subunits in BaeA, we initially calculated kinetic parameters for the enzymes using the concentrations of the dimers (and not the total concentrations of the individual putative active sites).

Wild-type BaeA and the three serine mutants all had the same k_(cat) value, suggesting that these serine residues are not involved in enzymatic turnover (Table 13). However, the variants in which Ser¹⁵ in Saro_2873 was mutated (3:S15A and 2:S14A/3:S15A) had K_(M) values that were 5 to 6-fold higher than that of the wild-type enzyme, suggesting that these variants bound the substrate weaker than the wild-type enzyme. The 2:S14A variant had the same K_(M) value as the wild-type enzyme, which, along with the lack of an effect on k_(cat), implies that Ser¹⁴ in Saro_2872 is not involved in catalysis by BaeA.

TABLE 13 Kinetic parameters for the enzymatic conversion of β(R)-MPHPV into GS-HPV k_(cat) K_(M) kc_(at)/K_(M) Protein (s⁻¹) (μM) (mM⁻¹s⁻¹) BaeA 2.9 ± 0.3 20 ± 3 150 ± 30  BaeA (2:S14A) 2.8 ± 0.2 22 ± 3 130 ± 20  BaeA (3:S15A) 2.4 ± 0.3 100 ± 20 23 ± 6  BaeA 2.8 ± 0.5 120 ± 30 23 ± 8  (2:S14A/3:S15A) BaeA (3:N14A) 0.14 ± 0.02 250 ± 60 0.5 ± 0.2 LigE 0.68 ± 0.06  7 ± 1 100 ± 20 

Analysis of the structure of LigF from Sphingobium sp. SYK-6 (PDB 4xt0) shows that the side-chain amide nitrogen of Asn¹³ is within hydrogen-bonding distance (3.3 Å) of the bound glutathione thiol group; an analogous asparagine is present in all of the previously characterized LigF enzymes, and in Saro_2873 (Asn¹⁴) (FIG. 29). (Saro_2872 has an Ala in this position (FIG. 29).) Active site asparagine residues are known or predicted to be involved in catalysis in other glutathione S-transferases. We mutated Asn¹⁴ in Saro_2873 into Ala and found that the resulting BaeA variant (3:N14A) had a k_(cat) value ˜20-fold lower and a K_(M) value ˜12.5-fold higher than wild-type BaeA (Table 13), suggesting that this residue is critical in both substrate binding and turnover in BaeA. We assume that mutation of this residue only affects the active site of the Saro_2873 subunit and does not have any long range effects on the active site of Saro_2872 or the overall folding or structure of the dimer; indeed, all mutant versions of BaeA used in this study ran as dimers in gel permeation chromatography (FIG. 27), similar to the wild-type enzyme, suggesting that the mutations did not affect the overall folding of the proteins or the binding between subunits.

The fact that mutation of a single residue in the Saro_2873 subunit had such a dramatic effect on the overall catalysis of BaeA suggests that Saro_2873 is the catalytically dominant subunit of the dimer, and that, if the Saro_2872 subunit has any activity in BaeA, it is <˜5% of the activity of the Saro_2873 subunit.

Catalytic Comparison of BaeA and LigE.

To directly compare catalysis between BaeA and LigE (Saro_2405), we also analyzed recombinant LigE in our in vitro reaction system. We found that LigE had a ˜4-fold lower k_(cat) value and a ˜3-fold lower K_(M) value than BaeA, leading to a catalytic efficiency (k_(cat)/K_(M)) for LigE that is slightly lower than that of BaeA (Table 13).

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EXEMPLARY VERSIONS OF THE INVENTION

Various exemplary versions of the invention are as follows.

Version 1: A method of processing lignin, comprising contacting lignin comprising β-O-4 ether linkages in vitro with:

-   -   a dehydrogenase comprising at least one of LigD, LigO, LigN, and         LigL;     -   a β-etherase comprising at least one of LigE, LigF, LigP, and an         enzyme comprising a first polypeptide having an amino acid         sequence of SEQ ID NO:40 or an amino acid sequence at least         about 95% identical thereto and a second polypeptide having an         amino acid sequence of SEQ ID NO:42 or an amino acid sequence at         least about 95% identical thereto; and     -   a glutathione lyase comprising any one or more of LigG and a         non-stereospecific glutathione lyase comprising an amino acid         sequence at least about 80%, 85%, 90%, or 95% identical to any         of:     -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ         ID NO:26 (ecYghU);     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU);     -   SEQ ID NO:30 (ecYfcG);     -   SEQ ID NO:32 (ssYghU);     -   SEQ ID NO:34 (GST3); and     -   SEQ ID NO:36 (PcUre2pB1).

Version 2. The method of version 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase.

Version 3. The method of version 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU);     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU);     -   SEQ ID NO:30 (ecYfcG);     -   SEQ ID NO:32 (ssYghU);     -   SEQ ID NO:34 (GST3); and     -   SEQ ID NO:36 (PcUre2pB1).

Version 4. The method of version 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 5. The method of version 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 90% or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 6. The method of any one of versions 1-5, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, or all of:

-   -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).

Version 7. The method of any one of versions 1-5, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of:

-   -   asparagine or a conservative variant of asparagine at a position         corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu));     -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

Version 8. The method of any one of versions 1-8, wherein the contacting occurs in the presence of a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG).

Version 9. The method of version 8, wherein the GSH reductase comprises an amino acid sequence at least about 95% identical to SEQ ID NO:38 (AvGR).

Version 10. The method of any one of versions 1-9, wherein the contacting releases at least one of a monomeric phenylpropanoid unit and a monomeric flavone.

Version 11. The method of any one of versions 1-10, wherein the contacting releases at least one of a monomeric guaiacyl phenylpropanoid unit, a monomeric syringyl phenylpropanoid unit, a monomeric p-hydroxyphenyl phenylpropanoid unit, and a monomeric tricin unit.

Version 12. The method of any one of versions 1-11, wherein the lignin comprises an average molecular weight (MW) of from about 600 to about 20,000.

Version 13. A composition, comprising:

-   -   lignin comprising β-O-4 ether linkages;     -   a dehydrogenase comprising at least one of LigD, LigO, LigN, and         LigL;     -   a β-etherase comprising at least one of LigE, LigF, LigP, and an         enzyme comprising a first polypeptide having an amino acid         sequence of SEQ ID NO:40 or an amino acid sequence at least         about 95% identical thereto and a second polypeptide having an         amino acid sequence of SEQ ID NO:42 or an amino acid sequence at         least about 95% identical thereto; and     -   a glutathione lyase comprising any one or more of LigG and a         non-stereospecific glutathione lyase comprising an amino acid         sequence at least about 80%, 85%, 90%, or 95% identical to any         of:     -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU);     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU);     -   SEQ ID NO:30 (ecYfcG);     -   SEQ ID NO:32 (ssYghU);     -   SEQ ID NO:34 (GST3); and     -   SEQ ID NO:36 (PcUre2pB1).

Version 14. The composition of version 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase.

Version 15. The composition of version 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU);     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU);     -   SEQ ID NO:30 (ecYfcG);     -   SEQ ID NO:32 (ssYghU);     -   SEQ ID NO:34 (GST3); and     -   SEQ ID NO:36 (PcUre2pB1).

Version 16. The composition of version 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 17. The composition of version 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 90% or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 18. The composition of any one of versions 13-17, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, or all of:

-   -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).

Version 19. The composition of any one of versions 13-17, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of:

-   -   asparagine or a conservative variant of asparagine at a position         corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu));     -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

Version 20. The composition of any one of versions 13-19, further comprising a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG).

Version 21. The composition of version 20, wherein the GSH reductase comprises an amino acid sequence at least about 95% identical to SEQ ID NO:38 (AvGR).

Version 22. A method of chemical conversion, comprising contacting a first compound in vitro with a non-stereospecific glutathione lyase to yield a second compound, wherein:

-   -   the non-stereospecific glutathione lyase comprises an amino acid         sequence at least about 80%, 85%, 90%, or 95% identical to any         of:         -   SEQ ID NO:18 (NaGST_(Nu));         -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));         -   SEQ ID NO:22 (SYK6GST_(Nu));         -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));         -   SEQ ID NO:26 (ecYghU);         -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU);         -   SEQ ID NO:30 (ecYfcG);         -   SEQ ID NO:32 (ssYghU);         -   SEQ ID NO:34 (GST3); and         -   SEQ ID NO:36 (PcUre2pB1)     -   the first compound has a structure of Formula I or a salt         thereof:

wherein:

-   -   R¹, R², and R³ are each independently —H, —OH, —O-alkyl,         —O-lignin, or -lignin;     -   R⁴ is —H, —OH, —SH, —COOH, —SO₃H, or —O-lignin; and     -   SG is glutathione bound in an S or R configuration; and     -   the second compound has a structure of Formula II or a salt         thereof:

wherein R¹, R², R³, and R⁴ are as defined above.

-   -   Version 23. The method of version 22, wherein:         -   R¹ in Formula I and Formula II is —H or —OCH₃;         -   R² in Formula I and Formula II is —OH;         -   R³ in Formula I and Formula II is —H or —OCH₃; and         -   R⁴ in Formula I and Formula II is —OH.

Version 24. The method of any one of versions 22-23, wherein the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 25. The method of any one of versions 22-23, wherein the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 90% or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu));     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu));     -   SEQ ID NO:26 (ecYghU); and     -   residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).

Version 26. The method of any one of versions 22-25, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, or all of:

-   -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).

Version 27. The method of any one of versions 22-25, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of:

-   -   asparagine or a conservative variant of asparagine at a position         corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu));     -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

Version 28. The method of any of versions 22-27, wherein the contacting occurs in the presence of a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG).

Version 29. The method of version 28, wherein the GSH reductase comprises an amino acid sequence at least about 95% identical to SEQ ID NO:38 (AvGR).

Version 30. The method of any one of versions 22-29, further comprising contacting lignin comprising β-O-4 ether linkages in vitro with enzymes to generate the first compound, wherein the enzymes comprise:

-   -   a dehydrogenase comprising at least one of LigD, LigO, LigN, and         LigL; and     -   a β-etherase comprising at least one of LigE, LigF, LigP, and an         enzyme comprising a first polypeptide having an amino acid         sequence of SEQ ID NO:40 or an amino acid sequence at least         about 95% identical thereto and     -   a second polypeptide having an amino acid sequence of SEQ ID         NO:42 or an amino acid sequence at least about 95% identical         thereto.

Version 31. A recombinant non-stereospecific glutathione lyase comprising an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu)); and     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)).

Version 32. The glutathione lyase of version 31, comprising an amino acid sequence at least about 80%, 85%, 90%, or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu)); and     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)).

Version 33. The glutathione lyase of version 31, comprising an amino acid sequence at least about 90% or 95% identical to any of:

-   -   SEQ ID NO:18 (NaGST_(Nu));     -   residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu));     -   SEQ ID NO:22 (SYK6GST_(Nu)); and     -   residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)).

Version 34. The glutathione lyase of any one of versions 31-33, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, or all of:

-   -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).

Version 35. The glutathione lyase of any one of versions 31-33, wherein the non-stereospecific glutathione lyase comprises at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, at least ten, or all of:

-   -   asparagine or a conservative variant of asparagine at a position         corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu));     -   threonine or a conservative variant of threonine at a position         corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu));     -   asparagine or a conservative variant of asparagine at a position         corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamine or a conservative variant of glutamine at a position         corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu));     -   lysine, a conservative variant of lysine, arginine, or a         conservative variant of arginine at a position corresponding to         position 99 of SEQ ID NO:18 (NaGST_(Nu));     -   isoleucine or a conservative variant of isoleucine at a position         corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu));     -   glutamate or a conservative variant of glutamate at a position         corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu));     -   serine, threonine, a conservative variant of serine, or a         conservative variant of threonine at a position corresponding to         position 117 of SEQ ID NO:18 (NaGST_(Nu));     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu));     -   arginine or a conservative variant of arginine at a position         corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and     -   tyrosine or a conservative variant of tyrosine at a position         corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).

Version 36. The glutathione lyase of any one of versions 31-35, wherein the glutathione lyase comprises at least one non-native modification selected from the group consisting of an amino acid addition, an amino acid deletion, and an amino acid substitution. 

We claim:
 1. A method of processing lignin, comprising contacting lignin comprising β-O-4 ether linkages in vitro with: a dehydrogenase comprising at least one of LigD, LigO, LigN, and LigL; a β-etherase comprising at least one of LigE, LigF, LigP, and an enzyme comprising a first polypeptide having an amino acid sequence of SEQ ID NO:40 or an amino acid sequence at least about 95% identical thereto and a second polypeptide having an amino acid sequence of SEQ ID NO:42 or an amino acid sequence at least about 95% identical thereto; and a glutathione lyase comprising any one or more of LigG and a non-stereospecific glutathione lyase comprising an amino acid sequence at least about 80% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1).
 2. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase.
 3. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 80% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); and SEQ ID NO:36 (PcUre2pB1).
 4. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 90% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); and residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).
 5. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); and residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).
 6. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises at least four of: threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)); asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)); glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)); lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)); isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)); glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)); serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)); arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)).
 7. The method of claim 1, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises at least seven of: asparagine or a conservative variant of asparagine at a position corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu)); threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)); asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)); glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)); lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)); isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)); glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)); serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)); tyrosine or a conservative variant of tyrosine at a position corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu)); arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and tyrosine or a conservative variant of tyrosine at a position corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).
 8. The method of claim 1, wherein the contacting occurs in the presence of a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG).
 9. The method of claim 8, wherein the GSH reductase comprises an amino acid sequence at least about 95% identical to SEQ ID NO:38 (AvGR).
 10. The method of claim 1, wherein the contacting releases at least one of a monomeric phenylpropanoid unit and a monomeric flavone.
 11. The method of claim 1, wherein the contacting releases at least one of a monomeric guaiacyl phenylpropanoid unit, a monomeric syringyl phenylpropanoid unit, a monomeric p-hydroxyphenyl phenylpropanoid unit, and a monomeric tricin unit.
 12. The method of claim 1, wherein the lignin comprises an average molecular weight (MW) of from about 600 to about 20,000.
 13. A composition, comprising: lignin comprising β-O-4 ether linkages: a dehydrogenase comprising at least one of LigD, LigO, LigN, and LigL; a β-etherase comprising at least one of LigE, LigF, LigP, and an enzyme comprising a first polypeptide having an amino acid sequence of SEQ ID NO:40 or an amino acid sequence at least about 95% identical thereto and a second polypeptide having an amino acid sequence of SEQ ID NO:42 or an amino acid sequence at least about 95% identical thereto; and a glutathione lyase comprising any one or more of LigG and a non-stereospecific glutathione lyase comprising an amino acid sequence at least about 80% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); residues 21-313 of SEQ ID NO:28 (recombinant ecYghU); SEQ ID NO:30 (ecYfcG); SEQ ID NO:32 (ssYghU); SEQ ID NO:34 (GST3); and SEQ ID NO:36 (PcUre2pB1).
 14. The composition of claim 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase.
 15. The composition of claim 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises an amino acid sequence at least about 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu)); residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)); SEQ ID NO:26 (ecYghU); and residues 21-313 of SEQ ID NO:28 (recombinant ecYghU).
 16. The composition of claim 13, wherein the glutathione lyase comprises the non-stereospecific glutathione lyase and the non-stereospecific glutathione lyase comprises at least seven of: asparagine or a conservative variant of asparagine at a position corresponding to position 25 of SEQ ID NO:18 (NaGST_(Nu)); threonine or a conservative variant of threonine at a position corresponding to position 51 of SEQ ID NO:18 (NaGST_(Nu)); asparagine or a conservative variant of asparagine at a position corresponding to position 53 of SEQ ID NO:18 (NaGST_(Nu)); glutamine or a conservative variant of glutamine at a position corresponding to position 86 of SEQ ID NO:18 (NaGST_(Nu)); lysine, a conservative variant of lysine, arginine, or a conservative variant of arginine at a position corresponding to position 99 of SEQ ID NO:18 (NaGST_(Nu)); isoleucine or a conservative variant of isoleucine at a position corresponding to position 100 of SEQ ID NO:18 (NaGST_(Nu)); glutamate or a conservative variant of glutamate at a position corresponding to position 116 of SEQ ID NO:18 (NaGST_(Nu)); serine, threonine, a conservative variant of serine, or a conservative variant of threonine at a position corresponding to position 117 of SEQ ID NO:18 (NaGST_(Nu)); tyrosine or a conservative variant of tyrosine at a position corresponding to position 166 of SEQ ID NO:18 (NaGST_(Nu)); arginine or a conservative variant of arginine at a position corresponding to position 177 of SEQ ID NO:18 (NaGST_(Nu)); and tyrosine or a conservative variant of tyrosine at a position corresponding to position 224 of SEQ ID NO:18 (NaGST_(Nu)).
 17. The composition of claim 13, further comprising a glutathione (GSH) reductase that catalyzes reduction of glutathione disulfide (GSSG).
 18. The composition of claim 14, wherein the GSH reductase comprises an amino acid sequence at least about 95% identical to SEQ ID NO:38 (AvGR).
 19. A recombinant non-stereospecific glutathione lyase comprising an amino acid sequence at least about 80% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu); and residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu)).
 20. The glutathione lyase of claim 19, wherein the glutathione lyase comprises: an amino acid sequence at least about 95% identical to any of: SEQ ID NO:18 (NaGST_(Nu)); residues 21-313 of SEQ ID NO:20 (recombinant NaGST_(Nu)); SEQ ID NO:22 (SYK6GST_(Nu); and residues 21-324 of SEQ ID NO:24 (recombinant SYK6GST_(Nu); and at least one non-native modification selected from the group consisting of an amino acid addition, an amino acid deletion, and an amino acid substitution. 